Menelaus' wife, Helen, was abducted by Paris, who took her to Troy. The Greeks come together to help him get her back, and the Trojan war begins.
But years later, believing he had been wronged by Agamemnon, Menelaus' brother and leader of the Greeks, Achilles decides to pull out of the war.
This demoralises the Greeks, who begin to lose the war. So Patroclus, Achilles' 'cup bearer' (a euphemism for lover) decides to wear Achilles' armour and join the battle without telling him.
That day's fighting turns the tide in favour of the Greeks, but Patroclus dies on the field of battle.
His sacrifice will help change the course of the war since Achilles' fury explodes. And it will kill everyone who dares to cross its path.