“By Demon’s Will” follows Lena, a student finishing her Master’s dissertation about people who believe they are possessed by demons. At a psychiatric clinic, she interviews patients with that profile and ends up meeting Rose, who confesses to have set fire to her family’s house with her parents and brother inside, under the guidance of demons. Investigating Rose’s past, Lena will find a VHS tape containing images recorded at her brother’s birthday party, clues to what actually might have happened.