"Bedevilled" follows a woman named Hae-won who decides to travel to the island of Mudu in order to visit her childhood friend Bok-nam. We quickly find out Bok-nam is living in an abusive household, and the only thing that keeps her going is her love for her daughter, Yeon-hee. When Bok-nam begins to suspect her husband, Man-jong, has been sexually abusing Yeon-hee, she begs Hae-won for help. However, Hae-won refuses to get involved, forcing Bok-nam to try and escape with Yeon-hee on her own.
Bok-nam sadly gets caught by her husband, who attempts to punish her. Yeon-hee tries to intervene, but is accidentally pushed and killed by Man-jong, who pins Yeon-hee's death on his wife. Bok-nam snaps and begins killing everyone on the island and even goes after Hae-won, who she blames for Yeon-hee's death.