So the brother decides to help in brother, but really he just wants to defeat Troy because it is so powerful. So they gather up all their allies, which includes Achilles (Brad Pitt) So their big army arrives in Troy. But they see that it is difficult for them to attack since Troy is surrounded by a very large wall. In the meantime Achilles captures Paris's cousin and saves her from his leader. Achilles is the best warrior they have. During this capturing you are introduced to paris's brother, Hector, his older brother.
So Paris makes a deal to fight Helena's husband and they fight, but Paris is very injured and losing so he begs his brother to save him, even though it would be dishonoring. So Hector kills Helena's husband, which infuriates the king of Greece because he went against the rules. Also hecotr kills Achilles cousin, who dressed up like him, by accident. So Achilles comes to avenge him and kills Hector in a battle. In the meantime Achilles is falling in love with Paris's cousin.
So in the end Troy is defeated by a "trojan horse" which was a horse gifted to Troy filled with soldiers in hiding. Achilles was killed only because an arrow went through his, Achilles, heel weakening him. Paris, Helena and the cousin escape.