Sonozaki Shion escapes from her private school to return to Hinamizawa. During her time there, she meets Satoshi, who protected her from a group of thugs. She also begins borrowing her twin sister Sonozaki Mion's identity, eventually taking over her life as her paranoia increases. This arc reveals that because Mion and Shion switched occasionally, and that their own names were reversed, Shion (the twin we know as Mion) was given the demon tattoo instead of Mion (the twin we know as Shion).
The twin in Wata naga shi-hen who tortures everyone is actually Shion dressed as Mion. She imprisons Mion in the cellar the day after the Watanagashi Festival which means that all occurrences of "Mion" afterwards in Watanagashi-hen is Shion acting as her. Shion switches places with Mion one final time and tells her to go down the well and apologize to Satoshi, only to tase her at the edge of the well and causing her to fall to her death. Shion is later rescued by police, appearing as if she was kept prisoner as Mion, which leads everyone to believe Shion was the true culprit who fell down the well and broke her neck.
While tormented by her consciousness which, Shion continues her vendetta and stabs Keiichi in the stomach declaring that she succeeded in killing all she targeted for the responsibility of Satoshi’s disappearance as she laughs maniacally and asks Satoshi to praise her. When she tries to sneak into her apartment through her window to avoid police surveillance, she slips and falls. As she falls, she sees Satoshi’s smiling face in the moon and tearfully apologizes to him before falling to her death.
Recovering in the hospital, Keiichi hallucinates "Mion" torturing him and dies from an apparent heart attack.