When Kōichi Sasakibara transfers to his new school, he can sense something frightening in the atmosphere of his new class, a secret none of them will talk about. At the center is the beautiful girl Mei Misaki. Kōichi is immediately drawn to her mysterious aura, but then he begins to realize that no one else in the class is aware of her presence.
Two years ago, a pair of idols, Tsubasa Kazanari and Kanade Amō, collectively known as ZweiWing, fought against an alien race known as Noise using armor known as Symphogear. To protect a girl named Hibiki Tachibana, who got severely wounded by the Noise, Kanade sacrificed herself. Two years later, as Tsubasa has fought the Noise alone, Hibiki ends up gaining the same power as Kanade.
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.