King Acrisius of Argos (Donald Houston) imprisons his daughter Danae (Vida Taylor), jealous of her beauty. When Zeus impregnates her, Acrisius exiles her and her newborn son Perseus in a wooden chest on the sea. Zeus (Laurence Olivier) kills Acrisius and orders Poseidon (Jack Gwillim) to release the Kraken to destroy Argos. Danae and Perseus are washed ashore on Seriphos, where Perseus grows into handsome Harry Hamlin.

Calibos (Neil McCarthy), the spoiled son of Thetis (Maggie Smith), is betrothed to Princess Andromeda (Judi Bowker), but Calibos gets on the wrong side of Zeus and is turned into a satyr-like creature. His mother Thetis transports Perseus to Joppa, where he befriends a soldier named Thallo (Tim Piggott-Smith) and a poet named Ammon (Burgess Meredith) and learns that Andromeda, cursed, cannot marry unless her suitor answers a riddle created by Calibos. Zeus sends Perseus a helmet made by Athena (Susan Fleetwood) that renders him invisible, a magical sword from Aphrodite (Ursula Andress), and a shield from Hera (Claire Bloom). Perseus captures Pegasus and follows Calibos’s vulture to obtain the solution of the riddle, but he is nearly killed by Calibos and loses his helmet.

The next morning, he answers the riddle and obtains Andromeda’s hand in marriage. As Thetis cannot act against Zeus, Calibos demands she take action against the city of Joppa. When Queen Cassiopeia (Sian Phillips) declares Andromeda more beautiful than Thetis, an earthquake strikes and topples the head of Thetis from her temple statue. The head demands that Andromeda be sacrificed to the Kraken. Pegasus is captured by Calibos, but Zeus orders Athena to give Perseus her owl Bubo. She gets Hephaestus (Pat Roach) to build one out of gold, which leads Perseus and his supporters to the Stygian Witches (Flora Robson, Anna Manahan, and Freda Jackson).

Using their one eye, Perseus forces them to reveal that he can defeat the Kraken with the head of the Gorgon Medusa. On the Gorgon’s Island in the River Styx, at the edge of the Underworld, Perseus glimpses Medusa in the reflection off his shield and cuts off her head. Calibos sneaks into the camp, punctures the cloak hiding the Gorgon’s head, releasing giant scorpions. Perseus survives and kills the scorpions.

Perseus sends Bubo to rescue Pegasus. Andromeda is shackled to the cliffs outside Joffa and the Kraken appears. Perseus arrives on Pegasus and petrifies the Kraken with Medusa’s head. He frees Andromeda and marries her. They will live happily, Zeus affirms, rule wisely, and produce children. Constellations are placed in the sky to honor all concerned.

Ray Harryhausen, aside from creating all the fanciful creatures in the film, was co-producer. He maintained that Bubos was created before R2-D2, though they seem to speak the same language. Columbia pictures dropped the project as too expensive. Orion Pictures insisted on Arnold Schwarzenegger playing Perseus. But the producer said that there were too many lines for him. Finally, MGM agreed to do it right. They hired the most glittering band of topnotch British stars until Harry Potter, which was a great enhancement. Maggie Smith convinced Olivier to be in the movie and everybody else jumped on the bandwagon. Burgess Meredith replaced John Gielgud so it would not be entirely British. Desmond Davis was hired to direct because he had done Shakespeare.

Harry Hamlin was romantically involved with Ursula Andress and their son was born after the film was completed. Malcolm McDowell, Michael York, and Richard Chamberlain were runners-up for his role. He threatened to quit at one point unless he was allowed to decapitate Medusa as in the myth. He took the job to work with Sir Laurence Olivier. Ursula Andress has main title billing but only one line in the entire film. Scriptwriter Alan Beverley Cross, writer of Jason and the Argonauts, was married to Maggie Smith. The film was highly praised by both Siskel and Ebert, but most reviews were mixed, always mentioning how old fashioned a story it was, some thinking that was good, some bad. There are in fact no Titans in the movie.