Yukiteru has always been rejecting any offers to go with friends, and he just keeps writing a diary on his cell phone. He has an imaginary friend called Deus Ex Machina. However, Deus now wants to play a game with Yukiteru, a game of survival. There are twelve contestants, Yukiteru is one of them. The winner of the survival game will become the next Deus Ex Machina. In order to win the game, the contestants must use their diaries to eliminate one another. For Yukiteru to survive, he must team up with another diary holder, a girl by the name of Yuno.
Each player is assigned a number based on the order that Deus modified his or her diary. The diaries now record the future in the same manner their future selves would have written down events that have already happened; the only exception to this rule is when the holder reaches a "Dead End", a time when their lives would be finished without significant intervention. Each diary varies in properties and functions depending on the holder's personality, occupation and lifestyle. All the diaries have their shortcomings, some more so than others, and can be used without the diary owner's consent should they fall into another's hands. Because the diaries are written from the perspective of the holder's future self, the future can still be misinterpreted and the diary entries can then turn out to be inaccurate. Each of the diaries have become the diary holder's future, so if the diary itself is destroyed or broken, the holder essentially dies.