Agamemnon has taken the Trojan Chryseis prisoner and holds her for ransom. Her father is a priest of Apollo and prays for help. Apollo strikes the Greek army down with pestilence until Achilles and his myrmidons ae forced to lead the Greek army out of Troy. Agamemnon eventually returns Chryseis. Achilles fights for Agamemnon and the Greeks, but when Agamemnon offends Achilles by taking his trophy bride, Briseis, he leaves.
Odysseus basically takes over for Achilles and leads the Greek army. During a truce, Paris and Menelaus fight over Helen