A world where it has been established that people routinely swing between slightly different parallel worlds... Koyomi Takasaki, who lives with his mother following his parents' divorce, enrolls in a local university prep school. Due to the atmosphere concentrated on studies and his social awkwardness, Koyomi is unable to make friends. One day, he is suddenly approached by his classmate Kazune Takigawa, who tells him that she has travelled from the 85th world where she and Koyomi are lovers.

A world where it has been established that people routinely swing between slightly different parallel worlds... Koyomi Hidaka, who lives with his father following his parents' divorce, meets a girl named Shiori Satou at the research institute where his father works. They both have a little crush on each other, but everything changes when their parents remarry. Koyomi and Shiori, convinced that they will never be together, try to jump into another world where they won't be step-siblings... There's no meaning in a world without her.

In a tranquil neighborhood of Tokyo, seven students are avoiding going to school—hiding in their darkened bedrooms, unable to face their family and friends—until the moment they find the mirrors in their bedrooms are shining. At a single touch, they are pulled from their lonely lives into a wondrous castle straight out of a Grimm's fairy tale. This whimsical place, oddly lacking in food and running water but full of electrical sockets, is home to a petulant girl in a mask, named Wolf Queen and becomes their playground and refuge during school hours. Hidden within the walls, they're told there's a key that will grant one wish, as well as a set of clues to find it. But there's a catch: the key must be found by the end of the school year and they must leave the premises by five o'clock each day or else suffer a fatal end. As time passes, a devastating truth emerges: only those brave enough to share their stories will be saved. And so they begin to unlock each other's stories: how a boy is showered with more gadgets than love; how another suffers a painful and unexplained rejection, and how a girl lives in fear of her predatory stepfather. As they struggle to abide by the rules of the game, a moving story unfolds, of seven characters trapped in a cycle of misunderstanding and loneliness, who are ultimately set free by the power of friendship, empathy, and sacrifice.

Kousuke and Natsume have been friends since childhood, but as time goes on, the relationship between the two sixth graders seems strained as they keep avoiding each other. One day during the summer vacation, they visit a housing complex that is scheduled to be demolished. Having grown up there, the place holds a lot of memories, but while playing, they suddenly get caught up in a mysterious phenomenon. When they regain consciousness, they see an entire ocean before them as the housing complex has drifted into a mysterious sea with Kousuke, Natsume, and others in it. Will they be able to return to their previous world? A summer farewell journey begins...