King Acrisius of Argos (Donald Houston) imprisons his daughter DanaÃ« (Vida Taylor), jealous of her beauty. When the god Zeus (Laurence Olivier) impregnates her, Acrisius sends his daughter and his newborn grandson Perseus to sea in a wooden chest. In retribution, Zeus kills Acrisius and orders Poseidon (Jack Gwillim) to release the last of the Titans, a gigantic sea creature called the Kraken, to destroy Argos. Meanwhile, DanaÃ« and Perseus safely float to the island of Seriphos, where Perseus grows to adulthood.
Calibos (Neil McCarthy), son of the sea goddess Thetis (Maggie Smith), is a young man engaged to marry Princess Andromeda (Judi Bowker), the daughter of Queen Cassiopeia (SiÃ¢n Phillips) and heir to the rich city of Joppa; but for committing several atrocities against Zeus, including hunting and destroying Zeus's sacred flying horses (excepting only Pegasus), Zeus transforms Calibos into a monstrous satyr and he is exiled by his people. In revenge, Thetis transports an adult Perseus (Harry Hamlin) from Seriphos to an abandoned amphitheatre in Joppa, where he is befriended by an elderly poet named Ammon (Burgess Meredith) and learns that Andromeda is under a curse and cannot marry unless her suitor successfully answers a riddle, whose failures are burned at the stake. In order to aid his son, Zeus sends Perseus a god-crafted helmet from Athena (Susan Fleetwood) which makes its wearer invisible, a magical sword from Aphrodite (Ursula Andress), and a shield from Hera (Claire Bloom). Perseus, wearing the helmet, captures Pegasus and follows Andromeda to learn the next riddle. Perseus is nearly killed by Calibos but manages to sever one of Calibos' hands, losing his helmet in the process.
Perseus befriends Thallo (Tim Pigott-Smith) and presents himself as suitor and correctly answers the riddle, presenting the severed hand of Calibos. Perseus wins Andromeda's hand in marriage. Calibos, finding that Thetis cannot act against Perseus, instead demands that she take vengeance on Joppa. At the wedding, Queen Cassiopeia compares Andromeda's beauty to that of Thetis herself, whereupon Thetis demands Andromeda be sacrificed to the Kraken on pain of Joppa's destruction.




The gorgon Medusa
Perseus seeks a way to defeat the Kraken, while Pegasus is captured by Calibos and his men. Zeus commands Athena to give Perseus her owl Bubo; but she orders Hephaestus (Pat Roach) to build a golden replica of Bubo instead, who leads Perseus to the Stygian Witches (Flora Robson, Anna Manahan, and Freda Jackson). By taking their magic eye Perseus forces them to reveal that the only way to defeat the Kraken is by using the head of Medusa the Gorgon, who lives on an island in the River Styx at the edge of the Underworld. The next day, the group continues on their journey without Andromeda and Ammon, who return to Joppa.
On the Gorgon's island with three soldiers by his side, Perseus fights Medusa's guardian, a two-headed dog named Dioskilos, who kills one of his companions but Perseus intervenes in the nick of time and kills the beast. Perseus leads his two remaining allies into the Gorgon's lair. His two other companions die on encounter with Medusa herself; she shoots one of the soldiers with an arrow and turns the other to stone. Perseus uses the reflective underside of his shield to deceive Medusa, decapitates her, and collects her head; but the shield is dissolved by her caustic blood. As Perseus and his party set to return, Calibos enters their camp and punctures the cloak carrying Medusa's head, causing her blood to spill and produce three giant scorpions called Scorpiochs. The scorpions attack and Thallo is able to kill one of them, but he is killed by Calibos himself. Perseus slays the other two scorpions and thereafter kills Calibos and grieves Thallo's death.




The Kraken comes to claim Andromeda
Weakened by his struggle, Perseus sends Bubo to rescue Pegasus, who is being held prisoner by Calibos' henchmen. Bubo succeeds in his task and manages to destroy Calibos' camp as well. Perseus reaches the amphitheatre in Joppa, but collapses unconscious from exhaustion. Andromeda is shackled to the sea cliffs outside Joppa, and the Kraken itself is summoned. Bubo diverts the Kraken's attention until Perseus, whose strength was secretly re-restored by Zeus before the Kraken was released, appears on Pegasus. In the subsequent battle, Perseus petrifies the Kraken with Medusa's head, causing it to crumble to pieces. He then tosses the head into the sea, frees Andromeda, and marries her.
The gods predict that Perseus and Andromeda will live happily, rule wisely, and produce children, and Zeus forbids the other gods to pursue vengeance against them. The constellations of Perseus, Andromeda, Pegasus, and Cassiopeia are created in their honor.
Where is the amphitheatre?
Joppa