Yukiteru "Yuki" Amano is a loner who spends most of his time writing a diary on his cell phone or talking with his imaginary friends Deus Ex Machina, the God of Time and Space, and Muru Muru, Deus's servant. One day, Deus gives Yuki a special diary, mentioning something about a game. Strangely, Yuki's new diary has entries in it spanning the next 90 days—entries about the future that come unnervingly true. Deus, the not-so-imaginary God of Time and Space, then forces Yuki to participate in a battle royal with eleven other people, each of whom also has a diary that can predict the future in some unique way. The rules of this "Diary Game" are simple: the contestants must try to find and kill all the other contestants before the world ends on Day 90, and only the last one standing can prevent the Apocalypse and become the new God of Time and Space.

Yuki initially has no desire to win the game, but after Deus informs the other contestants that Yuki, as the "1st" Diary owner, is his favorite to win the game, he is forced to team up with the 2nd Diary owner—his obsessive stalker, Yuno Gasai—in order to survive the constant attempts on his life by the other diary users, who see him as the biggest threat. Although Yuki only feigns affection for Yuno to manipulate her into protecting him, he soon becomes too afraid to spurn her after witnessing her psychopathic obsession with their "Happy End" (a prediction in Yuno's diary that she and Yuki will "become one" on July 28, the 90th day of the game) and the terrifying methods she employs to remove anything and anyone she perceives as a threat to it. Yet despite Yuno's apparent insanity and the mounting evidence that she is hiding something from Yuki, his declarations of love become increasingly sincere over the course of the game. Even as he learns that he cannot trust any of the other contestants, Yuki continues to trust Yuno because her feelings for him are genuine and because he is unwilling to personally stoop to the dirty tactics the competition resorts to in order to survive.

From Wikipedia