Three powerful Olympians, the brothers Zeus (Liam Neeson), Poseidon (Danny Huston), and Hades (Ralph Fiennes), once fought valiantly against the Titans. After their victory, Hades crafted the fearsome Kraken, a creature that would come to play a pivotal role in their new dominion. While Zeus took on the responsibility of humanity and ruled over them, Poseidon claimed the seas. However, Hades, tricked by Zeus, was left to govern the gloomy Underworld.
As the years passed, Hades learned that he could draw power not just from the worship of mortals but from their fear. A fateful day arrives when fisherman Spyros discovers a coffin in the depths of the ocean, revealing baby Perseus (Sam Worthington) and his deceased mother, Danae. Raising Perseus as his own, Spyros's fate unravels when Hades orchestrates a massacre in retaliation against humans who defy the gods, leading to the tragic loss of Spyros's family, leaving only Perseus to survive.
The soldiers taking Perseus back to Argos witness a feast in honor of their homecoming, where King Kepheus (Vincent Regan) and Queen Cassiopeia boast about their family, drawing unwanted attention from Hades. In a dramatic appearance, he persuades Zeus to punish humanity for their insolence. Hades avenges his anger by killing the remaining soldiers, whilst also aging Queen Cassiopeia into oblivion. He threatens that if Princess Andromeda (Alexa Davalos) is not sacrificed to the Kraken within ten days, Argos will face annihilation.
In the Underworld, Hermes (Alexander Siddig), the divine messenger, reveals the existence of Perseus to Zeus but suggests the possibility of offering him protection. However, Zeus coldly decides to abandon his son to his destiny among mortals. In his imprisonment, Perseus meets Io, a tragic figure cursed with eternal youth. She recounts the tale of Zeus's transgression against Acrisius, the former king of Argos, leading to heartbreak and vengeance.
Determined to slay the Kraken and weaken Hades, Perseus sets forth on a perilous quest. He is joined by brave soldiers from Argos, including the monster-hunters Ixas (Hans Matheson) and Kucuk (Mouloud Achour), along with Io. Meanwhile, Hades confers with Calibos, formerly Acrisius, revealing his intent to eradicate Perseus, whom he sees as a growing threat.
During their journey, Perseus and his comrades discover a magical sword forged in Olympus. However, Perseus chooses to reject the divine gifts, believing he must forge his own path. The journey is fraught with challenges, including an attack from Calibos that leads to the emergence of monstrous scorpions from his blood. Luckily, they are rescued by the Djinn, mystical beings who seek to thwart the gods' sinister plans.
Upon entering the eerie Garden of Stygia, Perseus learns from the Stygian Witches that the head of the fearsome Medusa (Natalia Vodianova) is key to defeating the Kraken. Although he is warned that the quest will lead to his demise, Perseus bravely proceeds. He receives a golden drachma from Zeus, enabling him to secure passage through the Underworld.
In the Witches’ lair, a devastating battle ensues, costing many lives under Medusa's petrifying gaze. But Perseus, against all odds, manages to behead the monster. Tragically, as he returns to confront Calibos, he finds that Io has fallen to the antagonist’s treachery. In a climactic confrontation, Perseus defeats Calibos and defeats the curse upon him.
Returning to Argos, Perseus learns of a plot to sacrifice Andromeda, which he desperately seeks to thwart. He triumphs over Hades' harpies and uses Medusa's head to petrify the Kraken before the cult leader meets his demise. In a final showdown, Hades scoffs at Perseus' bravery, but with raw resolve, Perseus uses the divine sword to banish him.
Ultimately, Perseus declines the offer of kingship and the offer to become a god from Zeus, preferring to remain among mortals. In a poignant act, Zeus resurrects Io, leading to a heartfelt reunion as Pegasus soars in the skies above. In the end, while Perseus embraces his human life, Zeus ominously warns that the threat from Hades will eventually return, heralding a future marked by darkness.
