Hajime's girlfriend was cheating on him with a friend. A devastated Hajime is approached by Mary, a novelist who lives in the room next to hers. Mary tells him she'll comfort him, and the two have sex. In the three days until she returns from her affair trip, the distance between them rapidly closes. They talk like lovers. They have sex like lovers. But their worlds are separated by a door. A different life that could have been had with one different choice. There are countless girlfriends, for every choice there is - the parallel world (multiverse) x Tachibana Mary story finally begins.