The main character is Amano Yukiteru. He is a shy guy who mostly keeps to himself. He likes writing down things he sees in his cellphone that acts as his diary. He’s very detached and prefers to only talk to his imaginary friend, Deus Ex Machina – The God of Time and Space. One day, he notices entries of his diary written in advance. It tells him of things occur in the future. He uses this advantage to cheat on tests and avoid bullies.
Things turned upside down for Yuki when a serial killer was suddenly out to get him. This killer is after his future diary and Yukiteru soon find himself trapped. He was then saved by Yuno Gasai, the pretty girl in his classroom who is also his stalker. She tells him of why the diary can predict the future and states that she and the killer also own diaries that tell the future.
Yuki soon learns that Deus is more than a figment of his imagination when he makes Yuki participate in a battle royale with eleven other people. The contestants are given special diaries that can predict the future, each diary possessing unique features that give it both advantages and disadvantages. Within the next 90 days, the contestants must try to survive until there is only one left standing. The winner will become the new God of Time and Space.
Read more here: Mirai Nikki