While mahjong is a game that is often played with family and friends, it is also a game that is played in the darkest corners of society. Nangou is a compulsive gambler who has accumulated debt over three million yen. In a last ditch attempt to clear his record, he decides to wager his life on a game of mahjong with the mafia. Unfortunately, as the game progresses, Nangou only moves further from the prize and closer to death. When all hope seems lost, the game parlor is suddenly intruded upon by Shigeru Akagi, a young boy on the run from the police. Desperate to turn the game around, Nangou hands the game over to Akagi after teaching him a few of the rules. The mafia can only smirk as Akagi sits down to play. However, they soon come to learn that Akagi is a natural-born gambler. An imposing figure who does not fear death. One who is destined to become a legend.

Yukiteru Amano is a shy middle schooler who regularly keeps track of what he does in his daily life by writing down all of his activities on his phone—a digital diary. Despite having no friends at school, Yukiteru is frequently seen talking to his supposedly imaginary friends Deus Ex Machina, the god of time and space; and Deus' servant, Mur Mur. One day, Yukiteru wakes up and discovers that certain events of his day are preemptively displayed on his cellphone. While initially dismissing it as a coincidence, he slowly realizes that the incidents written in his phone actually take place in the near future. After spending the day benefiting from this new asset, Yukiteru learns that his classmate Yuno Gasai possesses a similar diary. As the two team up to defeat an odd pursuer and head back to their respective homes, Deus Ex Machina explains that they—alongside 10 other contestants—have been drawn into a survival game whose victor will become the deity's successor. With no other options, Yukiteru and Yuno must use their cellphones—now called "Future Diaries"—to survive this unforgiving battle royale.

Kiri Haimura has an obsession with beautiful hair—specifically, cutting it. This bizarre trait is what leads him to seek out a rumored long-haired ghost who lives in an abandoned house on a hill. However, he finds not a ghost, but a beautiful girl with long flowing hair named Iwai Mushanokouji, the "Hair Queen," whose hair cannot be cut due to a family curse. Iwai explains that there is a death game surrounding her, and that if she is killed by a cursed object, aptly named a "Killing Good," the wielder gets their wish granted. After protecting Iwai from "Authors"—Killing Goods users—he learns there is in fact a Killing Good passed down in his own family: a pair of scissors used by his ancestor to commit murders. Naming the scissors "The Severing Crime Edge," he finds that he is able to cut Iwai's cursed hair with them, setting her free to live a normal life. However, many Authors seek to kill the Hair Queen, and Kiri will have to protect her in this lethal game of fate.

Ryouhei Arisu is a high school student with no ambitions and just wants to escape from what he feels is a meaningless reality. One night when he is with his two friends Daikichi Karube and Chouta Segawa, they see an abnormally large firework shoot into the sky, an event which signals a permanent change in their lives. Blinded by the incredibly bright explosion, the three find themselves transported to what is known as the Borderland. The only inhabitants of this world are participants of a deadly game where specific tasks must be accomplished if they wish to survive. As the three begin playing to determine how they can get back home, Ryouhei finds himself feeling alive for the very first time in his life.