After losing a bet, Keichi, Mion, Rena, Satoko and Rika parade around the town in costumes. When they come to a fork in the road, they decide to go to Yagouchi, a nearby abandoned village. Once there Keichii and Rena learn the mystery of a strange haunted quarry as they debate whether or not to check it out.

Realizing that he forgot to pack his bathing suit while rushing to meet his friends at the pool, Keiichi borrows a pair of trunks from Mion's uncle, who claims that these trunks have the magical ability to make women attracted to whoever writes their name on them and wears them for three hours. Later, Mion hears from her uncle that he had made a mistake in describing the abilities of the trunks, revealing that the person whose name is written on the briefs will fall in love with the wearer, meaning Keiichi, who had written his own name on them, will fall in love with himself.

Now that she and her friends are finally free from the eternally repeating tragedies of June 1983, Rika has a new lease on life. While biking with her friends, Rika carelessly collides with an oncoming truck and is apparently killed; when she awakens, she finds herself in another Hinamizawa. However, this world is quite different from other worlds in the past: Keiichi, Takano, Irie, and Hanyuu are nowhere to be found; she is no longer close friends with anybody, Satoko being particularly hostile; the dam project that threatens to flood the town is still underway; and none of the successive tragedies she witnessed countless times happened at all, meaning Satoshi and her own parents are still around. For the first time in her life, Rika is at a complete loss for what to do.

Rika finally makes contact with Hanyuu, who tells her that for her to return to her old world, she must find and dispose of a piece from her old world that prevents Hanyuu from entering this new world, which in turn prevents Rika from reincarnating—if this piece turns out to be a person, for instance, she must kill that person. She is initially eager to find the item and return to her old world, especially when she responds to Satoko's bullying by assaulting her. Shortly afterward, however, Rika experiences what life with a mother is like, and begins building a new friendship with Mion, Rena, and Satoshi, as well as making peace with Satoko. Rika now feels it's all a matter of deciding which world she would rather like to stay in until she learns that her mother, now the reincarnation of Oyashiro instead of Rika, is the item she must dispose of.

Rika becomes gradually more torn between remaining in this new world that knows no sin where everyone is living contently, or killing her mother and returning to her old world, and is devestated to learn that she does not have much time to act. Finally, Rika decides to fight for what she wants to live for. The next thing she knows, she awakens from a coma in her old world and happily reunites with her friends, the other world having apparently been a dream, though Rika concludes she must have killed her mother to regain consciousness nonetheless. Unbeknownst to her, however, the dream was developed by Hanyuu to teach Rika, as her friends help her realize, that people need to work past personal hardships in order to make life worth living.