Four years ago, during the dam incident, young police investigator Mamoru Akasaka comes to Hinamizawa to find leads to a child kidnapping. After hearing an enigmatic warning from a younger Rika, Mamoru becomes suspicious of some of the townsfolk.

Satoshi apologizes to Shion (still pretending to be Mion), and asks her to care for Satoko during the Watanagashi Festival. That night, his aunt is murdered; in order to cover for Satoshi, Shion reveals that she is actually Mion's twin sister. After the police questions her, she is brought back to the Sonozaki main estate to answer to her grandmother for all the trouble she has caused.

Upon talking with Ōishi, Shion finds out Satoshi has gone missing and decides to investigate further into the mysterious murders over the past four years. One day at the library, Shion meets Takano, who tells her more on the history of Hinamizawa and its dark past. As the story skips one year to the present day, similarities between Keiichi and Satoshi brings back memories of Satoshi to Shion.

Shion believes the Sonozaki family is connected to the acts of "demoning away" people in the village, especially that of Satoshi. She imprisons Mion in the dungeon and unintentionally kills her grandmother. Shion pretends to be Mion the next day and confronts Keiichi about the previous night's encounters, to which Keiichi still denies. Later, she finally discovers the ones responsible for Satoshi's disappearance.