Asai Takuma is a fourth year college student who is having a hard time finding a job and is desperate for the sake of his future with girlfriend, Shintani Risa. One day, he was asked out by his childhood friends Eiichi and Mamiko to dig out a time capsule that was buried 10 years ago. The time capsule contains a letter from Takuma himself in the past. However, Takuma also receives a letter from himself 10 years later with a "shocking future". He decides to collaborate with the future him to change his life.

The protagonist, who is looking for an apartment, accidentally opens a door to another world. There, she finds herself trapped in a huge, desolate building. It is a hellish place ruled by a certain monster, where neither going out nor dying is allowed!