A psychic researcher, Quintus, sends the mind of a prostitute, Diana, back in time to learn about her past-life experiences. In the Middle Ages, she shares the body of her past self, Helene, who is to die at dawn under suspicion of being a witch. At Diana's urging, Helene escapes prison, earning the attention of Livia (the witch for whose crimes Helene has been blamed) and of Satan himself. Via the psychic link between Diana and Helene, Quintus physically goes back in time to convince Helene to avoid her death, so he can witness the results of history changing. However, if Helene evades execution, her future selves, including Diana, will never come into existence, so she accepts her fated death. When Helene dies, her link with Diana disappears, leaving Quintus physically stranded in the past, much to Satan's amusement.