In Tokyo, a disaffected high school student named Light Yagami finds the "Death Note", a mysterious black notebook that can end anyone’s life as long as the writer knows both the target's true name and face. Initially disbelieving, Light quickly considers the renewed possibilities of such an ability at his disposal and annihilates high-profile Japanese criminals then targets international criminals. Five days after discovering the notebook, Light is visited by Ryuk, a "shinigami" and the Death Note's previous owner. Ryuk, invisible to anyone who has not touched the notebook, reveals that he dropped the notebook into the human world out of boredom and is amused by Light's actions.

As criminals around the world die from inexplicable accidents and heart attacks, the global media suggest that a single mastermind is responsible for the mysterious murders and name them "Kira" (キラ, the Japanese transliteration of the word "killer"). Hoping to apprehend Kira, Interpol requests the assistance of an enigmatic consulting detective, known as L, to assist in their investigation. Deducing that Kira is based in Japan, L tricks Light into revealing that he is in the Kanto region of Japan by manipulating him to kill a decoy. Furious, Light vows to kill L, whom he views as obstructing his plans. L deduces that Kira has inside knowledge of the Japanese police investigation, being led by Light's father, Soichiro Yagami. Under the suspicion that "Kira" could have family ties with members of the "Kira" investigation, L assigns a team of FBI agents to monitor the families of those connected with the investigation and L learns enough to designate Light as the prime suspect. Around this time, Light graduates from high school to college. L recruits Light into the Kira Task Force, with each trying to get the other to reveal crucial information.