As a "Who Done It?" style mystery, the film does not reveal Sister Mathieu as the villain until the end, but instead leads the audience to wonder if it is, in fact, Sister Gertrude, the protagonist of the story. Gertrude has recently recovered from a brain tumor removal and has since developed an addiction to heroin and morphine. She destroy's an elderly woman's dentures and then later intentionally waits until it is too late before giving her an injection that would save her life. To make matters worse, Gertrude then steals a diamond ring from the deceased woman and then takes it into town to sell it, where she also has promiscuous sex with a man in a bar. Still, despite all of this, Gertrude is riddled with guilt and confusion for her feelings and actions.
From here is when the crimes of Matthieu, Gertrude's roommate and lesbian lover, begin. She kills a priest by bashing in his skull and throws his body out a window. She murders another old man by suffocating him with cotton balls. At the end of the film, we learn that Mattieu bears a grudge against men, due to the abuse that she suffered when she was younger.
Full gallery: Sister Mathieu (Suor Omicidi)