Yukiteru "Yuki" Amano is a loner who never really interacts with people and prefers writing a diary on his cell phone with his imaginary friends Deus Ex Machina, the God of Time and Space, and Muru Muru, Deus's servant.
However, Yuki soon learns that Deus is real when he makes Yuki participate in a battle royal with eleven other people. Within this "Diary Game," the contestants are given special diaries that can predict the future, with each diary having unique features that give them both advantages and disadvantages.
The rules of the game are simple: the contestants have 90 days to find and kill all the other contestants, and the last one standing will become the new God of Time and Space.