In the year 2006, Light finds a mysterious notebook on the ground at his school, The cover reads "Death Note" and the book's instructions say that the human whose name is written in it will die. Light, skeptical of it's power, decides to test the Death Note on a criminal. Light decides to use its power to rid the world of evil, by killing criminals who he believes should die for their crimes. He soon meets Ryuk, a shinigami and the previous owner of the Death Note, who explains his reasons for dropping the Death Note in the human world. Ryuk tells Light that when it is his time to die, he will be the one to write Light's name in his notebook. Light then tells him of his goal of getting rid of all the evil people in the world, and ruling over it as God. Ryuk responds by saying "It's just as I thought, humans are so interesting!"
Light's killing reach a point where master detective L takes an interest in the case. The first of many conflicts between the killer and the detective begins when L places a scapegoat, Lind L. Tailor, to determine which country Kira, as Light is known, is currently in, starting with Japan. Light kills Tailor, believing him to be L, only for the real L to cut the broadcast, replacing it with an image of his signature, the letter 'L' in "Old English MT" or "Cloister Black" typeface. 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. 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, L leads a task force, which includes Light's father, Soichiro Yagami. As Light walks home from school, Ryuk tells him that he is neither on his side or L's, and also that he is being followed. Ryuk tells Light the true purpose of the Death Note, which is to increase the Shinigami's lifespan, and offers him Shinigami Eyes, allowing him to see the lifespan of other humans. Light refuses, but comes up with a plan to learn the FBI agent's name, allowing him to kill him.
A couple of days later, Light takes Yuri, a girl from school, on a date. Soon after, Kiichiro Osoreda boards the bus. Kiichiro hijacks it, as Light plans. Light writes a note to Yuri, saying that he is going to try and knock him out with a move his father taught him. The agent sees this and tells him it's too risky and to let him do it. After Light questions him, the agent shows Light his badge, giving Light his name, Raye Penber. Light then reaches into his pocket, dropping a note on the floor. Kiichiro sees Light going to pick it up and grabs it himself, throwing it back in Light's face simply because it was Light's date plans. The note was written on paper from the Death Note and so Kiichiro sees Ryuk. He shoots Ryuk, to no effect, and then jumps off the bus, only to be hit by a car.
On December 27, 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 labelled “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 threatens Penber by stating that non-compliance would lead to his death, the death of his love, and the deaths of his family. Light uses this threat to trick Penber into killing his fellow agents, before killing the man himself. Light's next threat, and Kira's next victim, is Raye Penber's fianceé, Naomi Misora. She believes that Kira was on the bus that Kiichiro hijacked, as Penber told her that he had to show someone his FBI badge. Light tells her that he is Soichiro Yagami's son and that he could tell his father what happened. She gives him a fake name, Shoko Maki, to protect herself and when he tries to kill her soon after, she survives. 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. She gives him her real name and writes it next to her time and cause of death on a piece of the Death Note. She leaves soon after he reveals to her that he's Kira, commiting suicide soon after. Because of his specifications, her body is never found.
Shortly after, Misa Amane becomes the second Kira. Light, having become friends with L in an attempt to learn his name and kill him, assists him in finding the second Kira, although it is Misa who finds him. Kira decides to use Misa as his eyes, as she has already made the Shinigami Eye deal with Rem, the previous owner of her Death Note.
Misa's recklessnes puts both Light and herself in danger and so he religuishes control of the Death Note. His Death Note is handed to a member of the Yotsuba Company, Kyosuke Higuchi, while Misa's, which she gave up after almost killing herself, ends up in a ditch. After relinguishing control of the Death Note, Light had no memory of being Kira and, when offered to by L, joined the task force in hunting Kira. 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 use Misa to kill L. When he met Rem, she threatened to kill him if he ever harmed Misa. By making her the prime suspect, Rem is forced to kill L and Watari, L's closest partner, to save Misa. This not only kills Rem, saving him from death should he threaten Misa's life again, but also puts him in the position of L, as the task force decides to keep the deaths secret.
At the age of 23, Light joins the Japanese Police Force's Intelligence Department in order to have better access to police information. Meanwhile, 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 and the SPK. Near, likewise, forms his own plan to finally expose Light as Kira. However, Mello intervenes and manages to cause Mikami to make a mistake, smashing Light's plans into the dust.
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.