Death Note

In Tokyo, brilliant yet disaffected high school student Light Yagami finds the "Death Note", a mysterious black notebook with rules that can end anyone's life in seconds if their name is written in it, as long as the writer knows both the target's true name and face. Light uses the notebook to kill high-profile criminals and is visited by Ryuk, a shinigami and the Death Note's original 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.
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"). Interpol requests the assistance of the enigmatic detective L to assist in their investigation. L tricks Light into revealing that he is in the Kanto region of Japan by manipulating him to kill a decoy. Light vows to kill L, whom he views as obstructing his plans. L deduces that Kira has inside knowledge of the Japanese police investigation, led by Light's father, Soichiro Yagami. L assigns a team of FBI agents to monitor the families of those connected with the investigation and designates Light as the prime suspect. Light graduates from high school to college. L recruits Light into the Kira Task Force.
Actress-model Misa Amane obtains a second Death Note from a shinigami named Rem and makes a deal for shinigami eyes, which reveal the names of anyone whose face she sees, at the cost of half her remaining lifespan. Deeply admiring Kira and seeking him to become her boyfriend, Misa uncovers Kira's identity as Light. Light uses Misa's love for him to manipulate her into discovering L's true name with her shinigami eyes. L deduces that Misa is likely the second Kira and detains her. Rem threatens to kill Light if he does not find a way to save Misa. Light arranges a scheme in which he and Misa temporarily lose their memories of the Death Note, and has Rem pass the Death Note to Kyosuke Higuchi of the Yotsuba Group.
With memories of the Death Note erased, Light joins the investigation and, together with L, deduces Higuchi's identity and arrests him. Light regains his memories and uses the Death Note to kill Higuchi and regain possession of the book. After restoring Misa's memories, Light instructs her to begin killing as Kira, causing L to suspect Misa. Rem realizes Light's plan to have Misa sacrifice herself to kill L. After Rem kills L, she disintegrates and Light obtains her Death Note. The task force agrees to have Light operate as the new L. The investigation stalls but crime rates continue to drop.
Five years later, cults worshiping Kira have risen. L's potential successors are introduced: Near and Mello. Mello joins the mafia whilst Near joins forces with the US government. Mello kidnaps Director Takimura, who is killed by Light. Mello kidnaps Light's sister and exchanges her for the Death Note, using it to kill almost all of Near's team. A shinigami named Sidoh goes to Earth to reclaim his notebook and ends up meeting and helping Mello. Light uses the notebook to find Mello's hideout, but Soichiro is killed in the mission. Mello and Near exchange information; Mello kidnaps Mogi and gives him to Near. Kira's supporters attack Near's group, but they escape. Shuichi Aizawa, one of the task force members, becomes suspicious of Light and meets with Near. As suspicion falls again on Misa, Light passes Misa's Death Note to Teru Mikami, a fervent Kira supporter, and appoints newscaster Kiyomi Takada as Kira's public spokesperson. Near has Mikami followed whilst Aizawa's suspicions are confirmed. Realizing that Takada is connected to Kira, Mello kidnaps her. Takada kills Mello but is then killed by Light. Near arranges a meeting between Light and the current Kira Task Force members. Light tries to have Mikami kill Near as well as all the task force members, but Mikami's Death Note fails to work, having been replaced with a decoy. Near proves Light is Kira by discovering Mikami had not written down Light's name. Light is severely wounded in a scuffle and begs Ryuk to write the names of everyone present. Ryuk instead writes down Light's name in his Death Note, as he had promised to do the day they met, and Light dies.
One year later, the world has returned to normal, and the task force members are conflicted over whether they made the right decision. Meanwhile, cults continue to worship Kira.

C-Kira (one-shot sequel)

Three years later, Near, now functioning as the new L, receives word that a new Kira has appeared. Hearing that the new Kira is randomly killing people, Near concludes that the new Kira is an attention-seeker and denounces the new Kira as "boring" and not worth catching. A shinigami named Midora approaches Ryuk and gives him an apple from the human realm, in a bet to see if a random human could become the new Kira, but Midora loses the bet when the human writes his own name in the Death Note after hearing Near's announcement. Ryuk tells Midora that no human would ever surpass Light as the new Kira.

a-Kira (one-shot sequel)

Another ten years later, Ryuk returns to Earth and gives the Death Note to Minoru Tanaka, the top-scoring student in Japan, hoping that he will follow in Light Yagami's footsteps. On explaining the rules to Minoru, Ryuk is surprised when he returns the notebook and tells him to return it and his memory of their encounter to him in two years' time. Two years later, Minoru receives the notebook back from Ryuk. Minoru reveals that he has no plans to use it himself. Instead, he plans to auction it off to the governments of the world, with Ryuk's help sending his offer out as "a-Kira", having waited two years until he was old enough to have a bank account to allow his plan to work. Elsewhere, Near (as L) is revealed to be developing technology meant to track Shinigami, although it is not yet advanced enough to be useful. After selling the Death Note to U.S. president Donald Trump for a sum that would ensure every Japanese citizen under the age of 60 would be financially set for life, Minoru relinquishes his ownership and memory of his plan to Ryuk, assuring his own anonymity. Trump is left unable to use the Death Note after the King of Death creates a new rule disallowing the Death Note to be sold, and he secretly returns it to Ryuk. Minoru collapses to the ground in the bank after withdrawing his savings. It is revealed that Ryuk wrote his name in the Death Note next to Light's. He longs for a human who will use the notebook for a longer period of time.