If you've seen Hercules the Disney movie (and not so much the original Clash of the Titans - or paid attention to Ancient History), then you can pretty much guess what will happen. And why is it called 'Clash of the Titans' when the Titans died in the opening scene??
Anyway, following advice from the 3 witches, Perseus and his men seek out Medusa. Io (Gemma Arterton) shows them the way, where they must travel to the Underworld, over the River Styx.
They enter her lair, but all the men fall - including that kid from About A Boy which is a relief. Perseus is left with Draco who gives the demi-god his chance by crushing her tail with a stalactite. Draco smiles as he is turned to stone. Perseus, using the reflection in his shield, spins and decapitates the monster. He exits the cave with her head.
Calibos (the monster king) appears and fatally stabs Io. Perseus and Calibos fight - and Perseus uses the sword from the gods to fight against the power of the Hades-inspired beast. Finally Perseus is victorious and Calibos turns back to a man again as he falls from the cliff.
Io dies, and Perseus realises that his time is up to save the people of Argos. Pegasus appears from the sky and Perseus jumps on to fly back and attack the Kracken.
Meanwhile the people of Argos have finished with their shopping and are trying their best to sacrifice the Princess to the monster. The Kracken emerges from the depths - but doesn't want just one little person, however good looking she is, and goes about flattening the city.
Perseus is attacked by the winged monkeys of Hades which steal the Gorgon's head. They race around the city, until the hero is helped by the comedy sidekick soldiers who returned from the fight earlier. Finally Perseus grabs the head and runs up the altar ramp next to the chained prisoner as the Kracken pops his Cloverfield style turtle head out of his shell, and sticks Medusa's head in front of the monster's eyes. Slowly, very slowly, the Kracken starts to turn to stone.
Hades appears, but not for long as Perseus throws the sword towards him and it strikes with the power of a thunderbolt (with a little help from Zeus), sending Hades back to the Underworld. The King has fallen in the background somewhere, and the princess falls a few thousand feet to the sea. Perseus dives after and rescues her. She wants him to stay, but he decides to leave.
He takes Pegasus to the ruined Zeus statue to look out where his foster parents died before. Zeus appears to congratulate his boy. Perseus still doesn't want to join him, so as a thank-you present, Io is brought back to life. And he can keep the horse.