Calibos (Neil McCarthy) is a young man who is the son of the sea goddess Thetis (Maggie Smith) and due to be married to Andromeda (Judi Bowker), daughter of Queen Cassiopeia (Siân Phillips) and heiress to the throne of Joppa; however, he has killed off every living thing in the Wells of the Moon (which he was entrusted to guard), among them Zeus' sacred herd of winged horses (except for Pegasus). To punish him, Zeus transforms Calibos into a monstrous satyr and has him exiled by his people. In retribution, Thetis transports the adult Perseus (Harry Hamlin) to an abandoned amphitheatre in Joppa where he encounters the aged poet Ammon (Burgess Meredith) from whom he learns that Andromeda is under a curse and is unable to marry unless her suitors can correctly answer a riddle; those who fail are burned at the stake. To aid his son, Zeus sends Perseus an invisibility helmet from Athena (Susan Fleetwood), a magic sword from Aphrodite (Ursula Andress) and a shield from Hera (Claire Bloom). Wearing the helmet, Perseus manages to capture Pegasus and follows Andromeda to learn the next riddle. Calibos nearly kills Perseus, but he escapes, losing his helmet in the process and also managing to sever Calibos' hand.