Laomedon' son, Ganymede, was kidnapped by Zeus, who had fallen in love with the beautiful boy. Laomedon grieved for his son. Sympathetic, Zeus sent Hermes with two horses so swift they could run over water. Hermes also assured Laomedon that Ganymede was immortal and would be the cupbearer for the gods, a position of much distinction.
Poseidon and Apollo, having offended Zeus, were sent to serve King Laomedon. He had them build huge walls around the city and promised to reward them well, a promise he then refused to fulfill. In vengeance, before the Trojan War, Poseidon sent a sea monster to attack Troy.
Laomedon planned on sacrificing Hesione to Poseidon in the hope of appeasing him. Heracles (along with Oicles and Telamon) rescued her at the last minute and killed both the monster and Laomedon and Laomedon's sons, save Ganymede, who was on Mt. Olympus, and Podarge, who saved his own life by giving Heracles a golden veil Hesione had made. Telamon took Hesione as a war prize and she gave birth to Teucer by him.
Note: In a popular alternate version of this story, Laomedon is replaced by King Tros and Heracles kills the sea monster but not him.