How does Achilles find out why there is a plague on the Greeks?

Achilles discovers the cause of the plague on the Greek camp through a consultation with the seer Calchas. Calchas reveals that the plague is a punishment from Apollo because Agamemnon has dishonored the priest Chryses by refusing to return his daughter, Chryseis. To appease Apollo and end the plague, Agamemnon must return Chryseis and offer a sacrifice to the god. This revelation sets off a chain of events that heightens the conflict between Achilles and Agamemnon.