Yukiteru Amano is a loner who spends his time writing in his mobile diary and talking to an imaginary friend, Deus Ex Machina. However, one morning, he finds his diary entry for the day has been filled in already. Stranger yet, the events portrayed in the diary start coming true. Deus, who reveals he is an actual God, warns Yukiteru that if he loses his diary, he will die. As Yukiteru starts using the diary to his advantage, he is approached by a stalker named Yuno Gasai who also possesses a Future Diary that foretold his death at the hands of a serial killer, another Future Diary owner. Helping him evade the killer, Yuno helps Yukiteru throw a dart into the killer's phone, killing him. Afterwards, Deus explains the rules of the survival game to Yukiteru and the other diary owners, in which they must try to kill each other in order to win the right to succeed him.
