Light Yagami, the brilliant university student who has been using the supernatural Death Note to eliminate criminals as the vigilante Kira, accepts an invitation from the world’s greatest detective, L, to join the official task force hunting him down. Light sees this as the perfect opportunity to get close to his nemesis and discover his real name. At the funeral of his girlfriend, Shiori, Light is haunted by the shinigami Ryuk, the Death Note’s original owner, who whispers that his oath to catch Kira means he must catch himself. This dark irony sets the stage for an escalating battle of wits.

The conflict is upended by the arrival of a second Death Note, which falls from the sky and is discovered by Misa Amane, a popular and naive model. Misa, whose parents were murdered by a criminal that Kira killed, worships Light as a god and becomes the self-proclaimed Second Kira. Accompanied by a new, white-and-blue shinigami named Rem, who is protective of Misa, she uses her notebook to broadcast her own lethal judgments on live television. Unlike Light, Misa has made a deal with Rem for the shinigami eyes, granting her the ability to see any person's name and lifespan just by looking at them, at the cost of half her own remaining life. She uses this power to kill indiscriminately, forcing a confrontation. When Light arrives at the scene of her attack, Misa looks at him and immediately recognizes him as the original Kira.

Rather than eliminating her, Light manipulates Misa’s devotion to use her eyes as his ultimate weapon. He plans to have her look at L to learn his real name, allowing Light to finally kill him. However, L acts swiftly, deducing Misa is the Second Kira and capturing her. With Misa in custody and her notebook hidden, Light is forced into a desperate, high-risk gambit. He voluntarily turns himself in, but only after instructing Rem to bury his own Death Note in the woods. He then relinquishes ownership of his notebook, causing him to lose all his memories of being Kira. Confident in his own innocence, the memory-wiped Light helps L profile the imprisoned Misa, all while a third Kira, a newscaster named Kiyomi Takada, begins killing again using the hidden notebook.

As the investigation progresses, Light’s father, task force chief Soichiro Yagami, touches the recovered Death Note, seeing Ryuk for the first time and confirming its horrifying power. The case hinges on a rule written in the notebook: a user will die if they do not write a name within thirteen days. This rule, secretly a lie added by Rem to protect Misa, leads the task force to believe the original Kira must be dead, clearing Light and Misa of suspicion. But L is not convinced. He suspects the thirteen-day rule is a fabrication.

In the final act, Light is released and reclaims his buried notebook. The moment he touches it, his memories as Kira return. Now armed once more with his full knowledge and ambition, he orchestrates a deadly final scene. He commands Misa to retrieve a hidden scrap of the Death Note and orders Rem to kill L and his butler, Watari, knowing that if Rem extends Misa’s life by killing her pursuers, the shinigami will turn to dust. As Rem acts, L collapses from a heart attack, appearing to die as Light watches with a triumphant smile.

The triumph is an illusion. L had anticipated Light’s scheme. Days earlier, L wrote his own name in the Death Note, specifying a peaceful death by heart attack for a future date. This brilliant counter-means that when Rem wrote L’s name, she only killed herself, as L was already condemned to die. The heart attack L suffers is a controlled one, giving him just enough time to reveal the truth before he finally passes away. Exposed as Kira in front of his father and the entire task force, Light panics. He shoots Soichiro and tries to write the names of everyone present on a piece of the notebook hidden in his watch. Cornered and defeated, Light begs the shinigami Ryuk to kill his enemies. Instead, Ryuk, who has only been an amused observer, calmly writes Light Yagami’s name in his own Death Note. As Light dies in his horrified father’s arms, still insisting he was trying to create justice, L’s final gambit succeeds. The Death Notes are burned, the reign of Kira ends, and the world learns that the last name ever written belonged to the detective who sacrificed himself to win.