Before the Zeus (Liam Neeson) and the Gods ascended to Olympus, their parents, the Titans, ruled the Earth. Zeus began a war with the Titans and eventually conquered them by having his brother, Hades (Ralph Fiennes), create the Kraken, a beast powerful enough to banish the Titans. Zeus created man so that their love and prayers could sustain the immortality of the Gods and, upon establishing the hierarchy, banished Hades to the underworld.
An old fisherman, Spiros, (Pete Postlethwaite) is at sea when a coffin floats to the surface. He opens it and finds a dead woman and her newborn child. He takes the child and names him Perseus. Io (Gemma Arterton)
23 years later, Perseus (Sam Worthington) and his family are fishing in the sea near Argos but cannot capture any fish. Spiros curses the Gods and emphatically declares that someone needs to take a stand at their injustice and arrogance. The family sails near the port of Argos and witnesses the Argian army destroy a statue of Zeus. Before their eyes Hades appears and transforms into harpies and kills the soldiers. Hades then sees the fisherman’s boat and destroys it. Perseus tries to save his family but they drown before his eyes. A passing boat heading to Argos rescues him.
Hades goes to Olympus and convinces Zeus to allow him to turn the humans against each other. By rejecting the Gods, their very immortality is at stake. Zeus allows Hades to reign terror on the humans in order to drive them to prayer and beg the Gods for deliverance.
In Argos, Queen Cassiopia and her husband Cepheus host a banquet where they claim to be on par with the Gods. Cassiopia claims that their daughter, Andromeda (Alexa Davalos), is more beautiful than any of the Gods on Olympus and ends her speech by declaring that they are the new Gods. Perseus is brought in as the prisoner of Draco (Mads Mikkelsen) and witnesses Hades arrival. His arrival sucks most of the soldiers into the Underworld except for Perseus. Hades notes that Perseus is a demi-god and the son of Zeus, thus immune to Hades’ basic attacks. Perseus recognizes Hades as the being who killed his family and prepares to attack him but Io stops him, telling him that his time will come. Hades turns his attention to the royal family and lectures Cassiopia about her lack of respect and kills her by rapidly aging her. He then curses Andromeda and announces that in 10 Days the Kraken will come and destroy Argos…unless the Cepheus sacrifices Andromeda. Hades leaves and the surviving banquet members seize Perseus and imprison him.
Io visits Perseus in prison and tells him about how he was born. Acrisius, a former king of Argos, waged a war against the Gods and held Olympus under siege. Zeus came down from Olympus and impregnated Acrisius’s wife by masquerading as Acrisius. Acrisius, shamed by the Gods, killed his wife and put the just born Perseus into her coffin. He cursed Zeus and as he held his wife’s coffin over his head, Zeus struck Acrisius with a lightning bolt, transforming into the monster known as Calibos.
On Olympus, the other Gods warn Zeus not to trust Hades but Zeus silences them. Zeus is informed that Perseus, his son, is in Argos. Zeus claims that Perseus has no love for him and seemingly forsakes him. Hades is allowed to continue his reign of terror. Recognizing that Perseus hopes to kill him, Hades finds Calibos and gives him some of his underworld power in exchange for Calibos’ allegiance. Calibos accepts the offer and goes off to find and kill Perseus.
Draco tells Perseus that an honor guard is taking him to see the Stygian witches to find out how to stop the Kraken. Perseus, having no prior training as a soldier, arms up and is joined by 8 other warriors. Io decides to accompany them in order to continue watching over Perseus. As they travel through the woods, Draco trains Perseus and teaches him how to fight with a sword. Constantly being reminded that he is part God, Perseus insists that he will avenge his family as a man, hoping to honor the life he had. Draco ridicules him for his refusal to embrace his heritage and tells him that if there is a God inside him, he will need it to take on Hades. Perseus finds a special retractable sword, left by Hermes for Perseus to use in his quest. Perseus gives it to Draco to prove that he is serious about fighting as a man.
Perseus walks around the woods and encounters a herd of winged Pegasi (winged horses). As Io tells Perseus of their history and how no one has ever ridden one, a large black Pegasus lands in front of Perseus. Io calms it and allows Perseus to stroke it, before shrieks are heard from the camp. The black Pegasus flies away as Perseus races back to the camp.
Perseus returns to find Calibos killing several of his honor guard. He and Calibos fight continuously until Calibos bites Perseus’ arm and Perseus cuts off Calibos’ hand. Calibos flees and Perseus pursues with the rest of the honor guard. As they chase Calibos through the woods and into a dessert, they notice that the blood from Calibos’ stump is mutating. Before long, the group is surrounded by Underworld Scorpions, which attack and kill all but 6 members of the honor guard. The group manages to kill three scorpions before being surrounded by even bigger scorpions. Just as they are about to be killed, they are saved by a group of cursed soldiers known as the Djinn. The Djinn exert control over the scorpions and tame them. Just as Perseus thinks they are all safe, he collapses from Calibos’ bite.
Io tells Draco that the bite is infected with a strong poison from the underworld. She takes care of Perseus while Draco and his men discuss the Djinn. The Djinn were once soldiers who were cursed so that they could always do battle. They look like inhuman monsters made of bark, but could fashion new limbs from wood and live for hundreds of years to do battle. They realize that a Djinn has snuck into Perseus’ tent and think that it intends to burn Perseus to death with its blue flames. However, the Djinn tell Io that it wants to heal Perseus because it recognizes that Perseus could be their salvation from the tyranny of the Gods.
With Perseus healed, the group goes to the Stygian witches, a trio of hags who share one eye and see the Past, Present and Future. When Perseus refuses to make a sacrifice, they attack the members of the honor guard in order to eat them. Perseus quickly incapacitates them by stealing their eye. Promising to destroy their eye, Perseus extracts the needed information: In order to kill the Kraken, he must use Medusa, a creature which turns anything that looks into its face to stone. But the Stygian witches tell him that he will die if he pursues this course and that Argos will fall. Perseus tosses their eye onto the ground and leaves with the group.
Io tries to console Perseus, but Perseus isn’t troubled by the news of his impending demise. He leads the group to the entrance of the Underworld. Two comrades depart but leave Perseus a Shield made from the back of the Scorpions they killed. As Perseus goes to a stream to reflect, he comes across Zeus. Zeus offers Perseus a place by his side in Olympus but Perseus doesn’t accept the offer. Zeus wishes Perseus luck and gives Perseus a coin with which to bribe the ferryman of the Underworld, Charon. Zeus leaves and Perseus prepares to fight Medusa.
At the entrance of the Underworld, the Djinn who healed Perseus throws the coin into the river Styx. Charon arrives on his boat and the 5 warriors and Io board the ship. Io tells them that Medusa was a woman who tempted Poseidon. When Poseidon came to take her, she hid in the temple of Athena and begged the goddess to save her. Athena did nothing and was disgusted with Medusa, who was raped by Poseidon in Athena’s temple. Her disgust was so great that she turned Medusa into a monster who could turn any man into stone and banished her to the Underworld. Anyone may go to fight Medusa, but only men may enter her temple, keeping all women safe from Medusa’s cursed stare. Io trains Perseus in how to fight Medusa and the two almost share a kiss before being told they have arrived.
Outside of the cave leading into the temple, the soldiers take stock of their situation. Perseus thanks them for their courage and the men go in together to fight Medusa. Medusa, aware of their presence, kills three of the men almost instantly and critically wounds Draco with an arrow. Perseus and the Djinn are thrown over the edge of a cliff but manage to climb back up to continue the fight. Draco watches Perseus and the Djinn use a tag team effort to take Medusa down without turning to stone, which unravels when Medusa’s tail snags the Djinn. Instead of turning to stone, the Djinn is shown to be immune to her stare before igniting his chest to sacrifice himself and wound Medusa. Following suit, Draco uses his swords and weight to jump on a stalagmite which pins Medusa’s tail to the ground. Draco laughs at her frustration as he is killed and turned to stone. Medusa turns her attention to Perseus, who waits until he sees he reflection in his shield before beheading her with his sword. He bags the head and quickly leaves as Medusa’s still writhing body is dragged into hell.
Outside the temple, Io waits anxiously but is relieved when she sees Perseus succeeded. However, she is quickly stabbed from behind by Calibos, who followed the group into the Underworld to kill Perseus. Perseus recovers the Sword of Zeus and fights with Calibos. He stabs Calibos through the chest with the sword, which turns Calibos back into Acricius for his final moments. Acricius begs Perseus to not become like the Gods and dies. Perseus holds Io as she dies and she fades away in front of his eyes. At that moment, the Black Pegasus arrives and Perseus uses it to fly back to Argos and complete his mission.
Back in Argos, religious fervor overruns the city and their fear of the Kraken serves to strengthen Hades. The citizens over run the royal palace and seize Andromeda for sacrifice. The citizens bind her to a sacrificial rock and await the arrival of the Kraken. On Olympus, Zeus gives Hades permission to release the Kraken but once it is done, Hades reveals that Zeus has only been losing his power, not regaining it through the fear of the citizens of Argos. Hades, who resided in the Underworld, could only survive on their fear, so he executed a plan to overthrow Zeus. As Hades gloats, Zeus points out that Perseus is still on his way to Argos. Alarmed, Hades teleports to Argos to kill Perseus.
As the Kraken emerges to destroy Argos, Perseus arrives on the Pegasus. He flies to the Altar where Andromeda is being held captive and prepares to use Medusa’s head to kill the Kraken. However, at the last moment Hades appears and transforms into the harpies, stealing the head and forcing Perseus to pursue them through the skies above Argos. He eventually tackles and kills the Harpie carrying the head and lands on the stone behind Andromeda. As the Kraken moves in for the kill Perseus jumps in front of her and uses Medusa’s head to freeze the Kraken. As its body collapses, most of Argos is destroyed and the king is stabbed in the ensuing chaos. Hades materializes to attack Perseus but Perseus uses the sword of Zeus to summon a thunderbolt and blast Hades back into the Underworld. In the ensuing shock waves, Andromeda falls into the ocean but Perseus saves her before she can drown.
Perseus drags Andromeda to shore and tells her that she has to become Queen of Argos to give the people strength. She asks him to stay but Perseus tells her that he can’t. He rides the Pegasus to the area where his family dies and just stares into the ocean. Zeus visits him and once again offers Perseus a place on Olympus. Perseus again rejects the offer but Zeus tells him that the things Perseus did were legendary and that men will think of him as a God. Zeus rewards Perseus by resurrecting Io and he leaves the two together to return to Olympus.
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