THE UNDEAD

Quintus, a psychic researcher who has spent seven years in Tibet, wants to send someone back in time into a past life via hypnosis. He hires (for $500) a prostitute, Diana Love, and plans to send her into a trance over 48 hours so she can access her past life. Quintus' former professor is present to witness it.

Quintus puts Diana into a trance and sends her back into the Middle Ages, where she shares the body of her past self, Helene, who is in prison, sentenced to die at dawn under suspicion of being a witch. That night is the witches' Sabbath and, as tradition dictates, the townsfolk must kill any woman suspected of witchcraft to be safe for the rest of the year.

In her head, Helene can listen to Diana's voice. It advises her to seduce the cell's guard until she is able to knock him out with her chains and escape. The plan works out, and Helene escapes prison, earning the attention of Livia (the witch for whose crimes Helene has been blamed) and of Satan himself. Livia is in love with the knight Pendragon, who only has eyes for Helene. Jealous, Livia constantly tries to get Helene captured without Pendragon knowing. Meanwhile, in the professor's office, Quintus discovers that Diana has changed the past. However, this creates a new complication, because if Helene evades execution, her future selves, including Diana, will never come into existence. As a proof of that, bruises start appearing on Diana's body. To save her, Quintus uses the psychic link between Diana and Helene to physically go back in time. However, once in the past, he starts getting tempted by the idea of being able to witness the results of history changing by Diana's decision.

Meanwhile, Helene tries to survive through the night, knowing that if she succeeds, she will have a whole year to prove her innocence, as the witches' Sabbath occurs once every twelve months. Among the people helping her is Pendragon, the good witch Meg-Maud and the madman Smolkin. Moments before dawn, Quintus manages to reach her and explain the consequences of her continuing to live. Not knowing what to do, she tells Quintus to use his powers of suggestions to make her able to speak with her future selves. All of them have fulfilling lives and wish to keep on existing. After listening to them, Helene accepts her fated death and runs to the local executioner. Pendragon goes after her but is stopped by Livia, who tries to convince him to let Helene die. Enraged, Pendragon kills Livia and tries to save Helene but arrives too late. Moments before dawn, Helene dies at the hands of the executioner. Diana wakes up from her trance, having fully recovered. However, now that her link with Helene has disappeared, Quintus is left physically stranded in the past, much to Satan's amusement.

Helene chooses to accept her execution to preserve the existence of her future selves, and is killed at dawn. Diana awakens unharmed in the present, while Quintus is left stranded in the past.

The ending underscores the paradoxes and dangers of altering history through supernatural means; Helene’s sacrifice ensures her future incarnations, including Diana, can exist. Quintus’s ambition leads to his own undoing, trapped in the past as a caution against meddling with time and fate. The film closes the loop on its central experiment with an ironic twist worthy of its Faustian themes.