Himari Takakura passes away after going out on a trip with her brothers, twins Kanba and Shōma. She was, however, miraculously saved by a spirit in a penguin-shaped hat. In return for her extended life, the twins must search for an item known as the Penguin Drum with the aid of three penguins, which are visible only to the siblings.
Yukiteru (Yuki) Amano is a loner who never really interacts with people and prefers writing a diary on his cell phone, with his only companion being an imaginary friend named Deus Ex Machina, the God of Time and Space. However, Yuki soon learns that Deus is not a figment of his imagination but real when Deus makes him a participant in a battle royale with eleven others. Within this "Diary Game", the contestants are given special diaries that can predict the future, with each diary having unique features that gives it both advantages and disadvantages.
Tomoko Kuroki is about as socially dysfunctional as it is possible for a first year high school girl to be. We start the story when she realizes she has attended high school for three months without even talking to anyone at all. Not all is lost, however, since she is irrepressibly optimistic about her chances to turn it around and live a fulfilling high school life.