King Acrisius of Argos imprisons his daughter Danaë, jealous of her beauty. When the god Zeus impregnates her, Acrisius banishes his daughter and his newborn grandson Perseus to sea in a wooden chest. In retribution, Zeus kills Acrisius and orders Poseidon to release the last of the Titans, a gigantic sea monster called the Kraken, to destroy Argos. Danaë and Perseus safely float to the island of Seriphos, where Perseus grows to adulthood.
Calibos, the spoiled and rebellious son of the sea goddess Thetis, is betrothed to Princess Andromeda, daughter of Queen Cassiopeia of Joppa; but for committing several atrocities against Zeus, including destroying Zeus’s sacred flying horses (except for Pegasus), Zeus transforms Calibos into a deformed monstrous satyr-like creature. In revenge, Thetis transports an adult Perseus from Seriphos to an abandoned amphitheater in Joppa, where he befriends a soldier, Thallo, and an elderly poet named Ammon and learns that Andromeda is under a curse and cannot marry unless her suitor, upon the threat of execution if he fails, successfully answers a riddle concocted by Calibos. Zeus sends Perseus a god-crafted helmet from Athena which makes its wearer invisible, a magical sword from Aphrodite, and a shield from Hera. Perseus, wearing the helmet, captures Pegasus and follows Calibos’s giant vulture carrying off Andromeda’s spirit during her sleep to learn the next riddle. Perseus is discovered and nearly killed by Calibos, but manages to sever one of Calibos’s hands, losing his helmet in the process.
The next morning, Perseus presents himself as the next husband to be and correctly answers the riddle, winning Andromeda’s hand in marriage. Finding that Thetis cannot act against Perseus, Calibos instead demands that she takes vengeance on Joppa. At the wedding in Thetis’ temple, Queen Cassiopeia declares Andromeda’s beauty greater to that of Thetis herself, whereupon an earthquake shakes the temple, causing the head of the statue of Thetis to break off and crash to the floor. Thetis, using the statue’s head to speak through, demands Andromeda be sacrificed to the Kraken on pain of Joppa’s destruction. Perseus seeks a way to defeat the Kraken, but Pegasus is captured by Calibos and his men. Zeus commands Athena to give Perseus her owl Bubo, but she orders Hephaestus to build a golden replica of Bubo instead, who leads Perseus, Andromeda, Ammon, Thallo and some soldiers to the Stygian Witches. By taking their magic eye, Perseus forces them to reveal that the only way to defeat the Kraken is by using the head of the gorgon Medusa whose gaze can turn any living thing into stone, 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.
The Kraken comes to claim Andromeda.
On the Gorgon’s island, most of Perseus’ men are killed. Perseus fights and kills Medusa’s guardian, a two-headed dog named Dioskilos. Perseus then enters the Gorgon’s lair, where he uses the reflective underside of his shield to deceive Medusa, decapitate her, and collect 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. Calibos and the scorpions attack and kill Perseus’s remaining escorts, including Thallo, whose death Perseus mourns. Perseus overcomes the scorpions and thereafter kills Calibos.
Weakened by his struggle, Perseus sends Bubo to rescue Pegasus from Calibos’s henchmen and reaches the amphitheater in Joppa, where he collapses 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 restored by Zeus, 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.