Red Sonja comes to the port city of Venzia in the kingdom of Argos. After witnessing two sailing vessels crashing into one another, she rescues a young woman from a couple of street brigands. Moments later, a mute thug clobbers Sonja across the back of the head and brings her to the dungeon of a mage known as the Hidden One.

When Sonja awakens, she finds herself trapped in a strange maze of mirrors and illusions. She meets an frail old man, another prisoner, who explains that the mage and a nameless demon are both trapped inside this citadel, and the only way they can be freed is if a mortal woman solves the riddle of the maze. To aid her, the old man gives Sonja a bag of bone dust. The bag has the ability to conjure an army of the undead to aid her.

Sonja has no intention of helping either the demon or the mage, but she is determined to free herself, so she sets about solving the riddle. After struggling through horrific nightmares and visions, she finds the Hidden One and defeats him by summoning the army of the undead.

As she prepares to leave, she hears a voice crying for help. Inside of a chasm is a man who appears to be a high-lord warrior. Sonja prepares to free him until she realizes that he is actually the demon attempting to bewitch her. She slays the demon, then goes back to free the old man. The two leave the castle together.