Yukiteru Amano, a junior high student, has trouble making friends. He views himself as a spectator from the sidelines and often writes down what he sees in a cell phone diary. Tormented by solitude, Yukiteru interacts with his imaginary friends Deus Ex Machina, the God of Time and Space, and Muru Muru, Deus's servant. One day, Deus grants Yukiteru's cell phone the ability to record the future for the next 90 days. Yukiteru is then forced to participate in a battle royale with eleven other people, each of whom also have a diary that can predict the future in some unique way. The rules of the game are simple: before the world ends on day 90, the contestants must find and kill all the other contestants, and the last one standing will become Deus's successor.
The Future Diaries, are the namesake tool of the Future Diary manga, anime and drama series. They are objects (mostly cellphones) with the ability to predict the future, in several different ways according to their users. They are used in the Survival Game, that determinates who will earn the throne of God of Time & Space.