In the year 2003 (2006 in the anime), Light Yagami discovers a mysterious notebook on the ground at his school. Written on the cover are the words "Death Note". The Death Note's instructions state that if a human's name is written within it, that person shall die. Light is initially skeptical of the notebook's authenticity, but after experimenting with it, Light realizes that the Death Note is real.
After putting much thought into whether or not he should continue using the notebook, he ascertains that no one else will be able to bear the burden of improving the world. Light then meets the previous owner of the Death Note, a Shinigami named Ryuk. He explains his reasons for dropping the Death Note in the human world and when it is Light's time to die, he will be the one to write Light's name in his notebook. Light then tells him of his goal of becoming the "god of the new world". Ryuk responds by saying "It's just as I thought, humans are so interesting!"
Light's killings reach a monumental scale, attracting the attention of the master detective L. The first conflict between Light and L happens over the television, beginning with the broadcast of Lind L Tailor. L places Tailor as a decoy on television in order to trickLight. The network states that the broadcast is worldwide and that Tailor's statements (made in English) are being translated into Japanese by Yoshio Anderson. After Tailor says that he is "L," hereads a
Tailor on his TV, and kills him with a heart attack. Light thinks the ordeal is over, until the broadcast of Tailor is suddenly cut, and L's signature letter 'L' appears. Using a voice synthesizer, the real L announces that Lind L. Tailor was a decoy and a criminal on death row. He explains that Tailor's information was never made available to the public, so there was no way Kira could have killed him until now. L boldly taunts Kira, asking him to try to kill him, as Light stares at the TV angrily unamused. After several seconds, L learns that Kira needs the person's real name and
face to kill the victim,otherwise Kira would have already killed him. L announces that the video was aired only in the Kanto area of Japan as a ploy to determine the location of Kira. L, originally planning to air the video in other areas in Japan at different times, stated that it would not be necessary to continue broadcasting Tailor's footage. L says that he is now speaking directly to Kira and tells Kira that he will find him. Ryuk finds the scene before him to be quite entertaining. As Light watches the broadcast, he also declares that he will find L and he will eliminate him. The two announce their intentions, and both declare that they are "justice".
Soon after, now working with the Japanese police, L begins to lead a Task Force alongside Light's own father, Soichiro, in order to expose and capture Kira. As Light walks home from school, Ryuk tells him that he is neither on his side or L’s side. However, Ryuk feels like he should tell Light that during the last couple of days there has been a man following him. Light is surprised, but feels that the investigator doesn't know that Light is Kira. Still, Light is determined to find out the man's name without looking too suspicious.
Ryuk tells Light the purpose of the Death Note. A Shinigami gets extra life from killing humans with a Death Note, which is why a Shinigami can live for many years and cannot be killed in any other way. However, Shinigami are apathetic when it comes to the human world, and they sometimes forget to write down names for centuries. Then, Ryuk explains to Light that the Shinigami have eyes that allow them to see any human’s real name and life span over their head. In addition, a Shinigami can make a deal with the owner of the Death Note to receive the Shinigami Eye” in exchange for half of that human’s lifespan. Ryuk mentions that he can see Light’s lifespan, but isn’t going to tell him how many years he has left because he is neither on his or L’s side. Ryuk offers to make a deal with Light for the “Shinigami Eyes.” Light refuses the deal, stating that he needs to live in order to rule over his utopian world. He asks Ryuk if there is anything else that he should know about the Death Note, and Ryuk says probably not.
Suddenly, Light has an idea about how to find out the FBI agent’s name. He decides to use his stock criminals in order to find out how specific the details of death need to be.Light boards a bus with his date, Yuri and Light's follower takes a seat behind the couple. Soon after, a man boards the bus. The man pulls out a gun and points it to the driver, taking the bus hostage. The bus driver is forced to call Space Land (the amusement park the bus is headed to) and report the situation. The bus driver explains that he is being held at gunpoint before the busjacker takes the phone. The busjacker says that the driver is telling the truth and to bring the financial reports of Space Land to the bus stop that is before the stop prior to reaching Space Land; if they call the cops or do not comply, he will kill everyone on the bus. Light shows Yuri a note telling her that she should not worry, and that he will attack the busjackers hand that’s holding the gun. Light's follower sees the note and tells Light not to do that, and that he will handle the situation. Light begins writing on a piece of paper, and Light's follower tells him that he doesn’t have to write notes, because the busjacker can'thear their voice over the
sound of the bus. Light crumbles the paper and puts it in his pocket, then tells his follower that he can tell from his accent that he isn't Japanese. The man says that he is Japanese American, and Light replies by asking him if he can prove that he isn't the busjackers partner. Yuri reacts in disbelief and Light explains to her that a tactic criminals use is to have one criminal act while the other hides and waits in secret, appearing as a bystander. Light asks again if he is the busjackers partner. The man, thinking that Light is not Kira or he would have killed the criminal by now, decides that he has no choice; he takes out an FBI badge and shows it to Light, revealing that his name is Raye Penber Light says that he believes him, but that he doesn't know why the FBI is there in Japan. Light asks Penber if he has a gun, to which he responds yes. Light replies that he will leave the situation to himight puts his hand into his pocket, then takes it back out, causing his piece of paper to fall onto the floor. The busjacker watches as Light tries to pick it up, and yells at Light to not move. The busjacker picks up the paper and thinks that they are passing notes. Penber recognizes the paper as the one that Light showed to Yuri and becomes worried. Penber puts his hand into his jacket, seemingly to reach his gun. However, the busjacker laughs and throws the paper back at Light, believing it to be a note inviting someone to Space Land. The busjacker tells everyone to not make any sudden movements or else, then turns around and suddenly begins screaming in panic, asking what that “thing” is at the far back of the bus. He begins asking it when he appeared, and Ryuk replies by asking if he is talking to him. Ryuk asks the busjacker if he can see him, and the busjacker, appearing scared tells Ryuk to not move and that he will shoot him. Penber says that the busjacker is hallucinating, and warns everyone on the bus to get down. Ryuk, finally understanding Light's plan, notes that Light intended for the busjacker to pick up a piece of the Death Note. The busjacker begins shooting at Ryuk, and Ryuk says that bullets can not kill a Shinigami. As the busjacker continues to shoot, Ryuk notes that he told this to Light, and thinks it clever that Light had constructed this situation to reveal the name of the man that has been following him. Penber sees that the busjacker has used up all of his bullets, and prepares to act. However, the busjacker flees the bus and onto the road where he is immediately hit by a car.
Light begins exiting the bus, and finds Penber waiting for him. As Light exits, Penber tells Light that he is on a secret mission. Light says that he understands and will not tell anyone that they met, not even his father. Penber smiles before telling Light that he is leaving because things could get complicated if he is here when the police come.
Back at Light’s house, Ryuk asks him if he’s going to write Penber’s name down in the Death Note. Light tells him that it would look too suspicious if he does so immediately after they met. In addition, he wants Penber to show him the names and faces of every FBI agent sent to investigate the NPA On December 27, 2003 Light goes to an underground section of the Shinjuku Station. Ryuk asks Light what he plans to do, and Light says that he's going to show him the results of the experiments he's done over the last six days. Light reveals that he has been experimenting with the Death Note and has found several new ways to kill. Light figured out that he can write the date and cause of death in the Death Note without a name, and the Death Note will work when the name is added later. At 3:00 PM , while wearing a hood to obscure his identity, Light holds Penber hostage in an underground section of the Shinjuku Station. Light appears behind Penber (unaware that it is Light) and tells him that he is Kira, then instructs him to not turn around. Light says that he will prove his identity by killing Yonegoro Nusumi, an employee at a shop labeled “Cafeel” with a heart attack. The shop employee dies, and Light explains to Penber that Nusumi had raped women but escaped a court conviction due to a lack of evidence. Light says then says that he is purifying the world. Light says that he believes L has already told Penber that he can't kill without a name and face, and Light says that he can kill anyone in this station. Light asks Penber if he needs to prove his identity, and Penber responds that he believes the man is Kira. Light says that there must be someone more important there that would make a great hostage, and Penber asks if he means "her". Light tells Penber that he is correct, and explains that he has done a background check on his family. Light threatens Penber by stating that non-compliance would lead to his death, the death of his love, and the deaths of his family. Light asks if he's carrying a laptop, and if he has a file containing the names of all of the FBI agents. Penber says that he doesn't have a file like that,and Light hands him an envelope. He tells Penber to hold onto the envelope, take out the transmitter and put the earphones in. Over transmitter, Light leads Penber through a turnstile in the western entrance at 3:11 PM. He tells him not to look around when he gets on the train, and to remember that he is it Kira's mercy. He says to sit near a window and to waite for a seat to open if there are none available. Penber enters a train on the Yamanote Line at 3:13. Penber takes a seat on the train and Light tells him to answer a question, and not to lie, as if he does he will kill his fiancee. Light asks how many FBI agents there are, and Penber says that he thinks the are four groups, totaling twelve. Light says to call an agent that is ranked lower than him, and ask for the names and faces of all agents sent to Japan. Light states he needs to be able to hear the conversation. Penber uses his cell phone and calls an agent, and then asks him for the name and faces of the agents. The agent asks Penber why he doesn't ask his commander, and Light tells Penber to say that the connection between their computers is bad, and it would be better to use someone elses computer. Penber receives the FBI file at 3:21 PM, and Penber is the second FBI agent that receives it.Light tells Penber to take the contents out of the folder he gave him, and Penber takes out four envelopes with ten rectangular spaces cut out of the left side of each one. Light says to write down the names of the agents into the blocks on the folders. As Penber writes, Light tells him not to get off of the train when he's done, and that if he follows his instructions his fiance and family will not be killed. As Penber finishes, Light says to put the transmitter and envelope back into the folder and sit quietly for thirty minutes, then place the envelope above on the rack and leave the train when no one will notice. At 4:42 PM, Penber dies from a heart attack three seconds after he exits the train, and his last sight is of Light’s face. Penber appears shocked as he looks at Light's face and says "Light Yagami." Light watches as Penber dies, and says "Farewell, Raye Penber."
Light takes the files home and takes the pages of the Death Note out of their envelopes, revealing the times and causes of death for each agent. Each agent will receive the file and die at a different time.
Light's father holds a family meeting in their kitchen, three days before the New Year, and announces to Light,Sachiko and Sayu that he is working on the Kira case. Soichiro says that twelve FBI investigators were killed, and Light asks if they were killed by Kira. Soichiro says that it appears that way, and says that the Kira case is dangerous. He says that many officers have left the investigation and he could think of no reason to make them stay. Worried, Sachiko suggests that Soichiro quit the investigation, and Sayu also says that he should quit and that she doesn't want him to die. Soichiro tells them that he will not give up or back away from crime. Light tells his father that he thinks what he's doing is admirable, and says that he supports him. As Light walks from the kitchen, he tells his father that if anything happens to him he will personally put an end to Kira. Light shuts the door leading to the downstairs area of the house, and begins to climb the stairs leading to the second story. Ryuk tells Light that his acting is good, as well as his method of killing the FBI investigators.
Light's next threat comes from an unexpected person: Raye Penbers fianceé. After the sudden death of Raye, Naomi Misora decides to take part in the Kira case herself. Having been told by Raye that he showed his FBI badge to someone during a bus jacking, Naomi deduces that Kira must have been on the bus. In her attempts to contact the Japanes Task Force? comes across Light Yagami.Light uses his father's status to his advantage, telling Naomi that he is Chief Yagami's son, and slowly begins to win her trust. Cautious at first, she uses the false name Shoko Maki? in order to protect herself until she can reach L. Light finally gets Naomi to reveal her name by saying that he is a member of the Task Force and is actually working with L. Taking advantage of Naomi's emotional state, Light gives the possibility that she could join the Task Force and help catch her fiancé's murderer. Light says that he will need to see a form of identification before she can join, and Naomi shows him her drivers license. Using a piece of the Death Note, Light writes down Naomi's name, resulting in her death by suicide. Her body is never found due to the specifications made by Light in the Death Note.
On April 22nd in a broadcast on Sakura TV, "Kira" states that if criminal reports stop Kira will kill police officers and reporters, and then demands that the life of someone in charge of the Kira investigation be handed over--the Director's or L's. "Kira" says to choose one and reveal them in four days and that, because Kira knows the Directors face, there are no special requirements; however if they should choose L, he must appear on Sakura TV's 6 o'clock news and give a ten minute speech. "Kira" says that Kira will decide if the person on the news is L, and if Kira decides that he isn't, Kira will take the lives of several police officers as compensation. "Kira" says to please not lie as Kira does not wish to kill those without sin, and to think carefully over the next four days.Light Yagami watches the broadcast and laughs, amused, as Ryuk watches with a grin on his face. Light asks Ryuk if this is the work of another Shinigami, and Ryuk says that it likely is. Light responds positively to the idea that the owner of the Death Note is supportive of his ideals, then says that he is the only one who know that this new Kira is fake. Light decides that he needs to find a way to use this new Kira to his advantage, then says that the new Kira likely has the Shinigami Eyes based on how he killed the police officers, and is therefor more powerful. Light decides that he can use the new Kira to prove his own innocence and get rid of L at the same time, and then states that the new Kira's actions have also been damaging Kira's reputation and if he gets caught by the police it could be troubling for Light. Light says that the best course of action for him to take is to find a way to get on the Task Force so that he can keep an eye on L and the new Kira, and that it should be simple because L did not reject to him joining the Task Force earlier. Light states that if the new Kira ever messes up, he can just kill him and take the new Kira's notebook before L does, although it would be favorable if the new Kira just killed L in four days. Light decides to find a way to contact the new Kira without revealing his identity in order to take control of him.
A few days later, L decides that he wants Light to help the Task Force. L has Soichiro call Light and tell him to come immediately if he agrees to join them. Light arrives at the hotel and appears surprised at the small size of the Task Force, and L tells Light that there are a few outsiders who know what happens within the investigation and only one can contact L directly. Light thinks to himself that if everyone in the Task Force dies but one, the killer is revealed. Therefor, he must find out who the outsider with the information is. Light is told to look over the papers of the investigation and then watch the tape, and to not take notes or remove anything. As Light watches the tapes he begins thinking that he wants to throw up, and wonders if it's because he is the real Kira. After watching the tapes Light says that there may be someone else with Kira's abilities, citing the difference in criminals killed, as well as the fact that some were killed immediately after revealing their faces.
L reveals that he came to the same conclusion, and Light asks if L was testing him. L denies that he was, and tells Light that he was the only one who thought there could be a another Kira, and needed Light to think of the same idea without his opinion. Light realizes that L would have told him his plan regardless of what he told L, and that L set things up so that if he didn't say there was another Kira he would look suspicious, and if he did, he would help prove L's theory to the Task Force. L asks Light to help their case by pretending to be Kira, and tells Light that only he is able to play this role and tells him to begin writing a draft for the speech. When Light is through he asks L if his draft is good, and L says that it is, but if he doesn't remove the part about it being okay to kill L, he'll die. Light laughs and says that he was kidding, and that it's fine to remove it, and L gives the draft Aizawa to take care of. Later, the tape is created and sent to various TV stations.The Second Kira responds with videos which contain indirect, and some rather direct, comments about Death Notes, Shinigami and how they can meet. Light goes to Aoyama to find the Second Kira, but is unable to meet him. However, due to having Shinigami Eyes the Second Kira is able to identify Light as Kira due to being unable to see Light's life span. After finding out his address, the Second Kira arrives at Light's house, telling his mother and sister that she has his notebook. Light invites Misa to his room and asks Misa to explain her actions. Misa says that she wanted to meet Kira because her parents were murdered in front of her, and Kira killed the man who did it. Misa asks to be his girlfriend and partner in crime, admitting that she wouldn't even mind if he simply uses her. Light agrees, intending to kill her after she outlives her usefulness. However, Misa's Shinigami, Rem promises to kill him if he ever harms Misa.
Misa's reckless actions put both Light and Misa in danger, thus allowing L to finally corner Kira. Deciding to manipulate Rem, Light puts his master plan into action. He temporarily gives up his Death Note to Ryuk, erasing his memories about his time as Kira.
Rem, under instruction by Light to find someone who would use the notebook for selfish means, finds Kyosuke Higuchi of the Yotsuba Corporation.
Now unaware of the fact that he was the original Kira, Light accepts L's offer to join the investigation team to hunt down Higuchi, who has taken up the mantle of Kira. During the investigation of Yotsuba, Light works under the alias "Light Asahi". When Higuchi is finally captured, Light touches his Death Note, regaining his memories of being the original Kira. He then kills Higuchi by using the Death Note paper hidden in his watch to effectively become the owner of the notebook again.
Later, he is able to put Misa in danger again by having her act as Kira in his stead, prompting Rem to kill both Watari, L's closest partner, and L himself in order to save Misa. Shortly after L's death, Light is nominated by the Task Force (who are unaware that he is Kira) to act as L, deciding not to publicly reveal L's death. Light subsequently fills in the role of Watari as well. Now able to act as both Kira and L, Light continues his killing of criminals, all the while turning the Kira Investigation into a cold case.
About five years pass and Light's killings aren't limited to just Japan, but the entire world. The world falls into a dark age in which essentially everyone now lives under the law of Kira
At the age of 23, Light joins the Japanese Police Force's Intelligence Department in order to have better access to police information.
Near and Mello, the "true successors of L" eventually appear before him, trying to accomplish what L could not: exposing Kira's true identity and bringing him to justice. Near joins the Special Provision for Kira (or the SPK) for help in his endeavor, and Mello decides to use the Mafia for his investigation. Because of Mello's unscrupulous, criminal methods, Near sides with Light, not knowing that the latter is Kira. Mello is able to at one point take the notebook away from Light, but the combined efforts of Light and Near allow Light to regain the notebook at the cost of his own father's life (which Light would later explain to have been a necessary sacrifice). Once Near realizes that Light was responsible for the lack of progress in the case as the new L, he begins to suspect that Light may in fact be Kira. With a prosecutor named Teru Mikami acting as a new proxy, Light formulates a plan to eliminate not only Near but the surviving members of the task force. Near, likewise, forms his own plan to finally expose Light as Kira. However, Mello intervenes and manages to cause Mikami to make a mistake, derailing Light's plans.
Light, Mikami, the SPK and the Task Force all eventually meet at the Yellow Box warehouse, where Near uses Mikami to expose Light as Kira. After confessing and attempting to justify his actions to the people tasked with capturing him, Light tries to kill Near with the notebook clipping hidden in his watch, only to be shot by Touta Matsuda, a member of the task force.
Mikami commits suicide, creating a diversion for both the task force and the SPK. Although weakened, Light catches a second wind and manages to flee the warehouse. Near tell Shuichi Aziawa Kanzo Mogi, Hideki Ide and Matsuda that there is no reason to pursue Light given that there should be no more hidden Death Note pieces and Light's wounds would prevent him from getting far. Aizawa refuses to heed Near's orders and has his team follow Light. Meanwhile, Ryuk writes Light's name down when the latter finds another warehouse to hide in. As Light silently dies, an illusion of L appears before Light, mirroring a scene following L's defeat at Light's hands in episode 25.
At the end of Death Note Relight 2: L's Successor Aizawa and Matsuda are revealed to have found the warehouse Light has fled to; their expressions of shock imply that they have seen Light's body.