In modern day Japan, a university student named Light Yagami discovers a black notebook dropped by a Shinigami, or death god, named Ryuk. The notebook's instructions are clear: anyone whose name is written in it while the writer pictures their face will die. Initially dismissing it as a prank, Light tests the notebook on a bully harassing him, and when the bully dies, he is horrified to discover the power is real.

Terrified and overwhelmed by guilt, Light attempts to discard the notebook, but Ryuk, who in this version actively chose Light as the recipient, goads him into embracing its power. When Light's father, a police detective, is taken hostage by a criminal, Light uses the Death Note to save his life. This act convinces him to use the notebook to rid the world of evil, becoming a vigilante serial killer known to the public as Kira.

As criminals around the globe begin dying from mysterious heart attacks, the world's authorities turn to the enigmatic and world-renowned detective known only as L. L, a brilliant but eccentric figure, quickly deduces that Kira is a person in Japan and begins a high-stakes cat-and-mouse game to identify and capture him. The conflict forces Light to walk a tightrope, using the notebook's rules to kill while trying to stay one step ahead of L and the Japanese police task force, which includes his own father.

Unlike the original story, this Light is not a genius prodigy but an ordinary, ambitious young man with a close group of friends and a part-time job. His descent into becoming Kira is a gradual and tragic corruption, driven by a desire to create a just world after his mother was killed in an unsolved crime. The series also introduces significant changes to other characters. The cheerful idol Misa Amane, a devoted Kira supporter who obtains the Shinigami Eyes to see anyone's name and lifespan, plays a larger role and is the object of Light's genuine affection rather than just a tool. Furthermore, L is not working alone; he is in regular contact with his successor, the young genius Near, who is brought into the investigation much earlier than in previous versions.

The narrative accelerates through familiar arcs, including Light’s temporary loss of his memories and his alliance with L to pursue a new Kira, before diverging into an entirely original conclusion. The drama builds towards a final confrontation that redefines the relationship between Light, L, and Near, ultimately leading to a resolution that differs completely from the manga, anime, and films, as the series was promoted as having a new ending from the start.