Atreus, (Thyestes' brother and King of Mycenae) vowed to sacrifice his best lamb to Artemis. Upon searching his flock, however, Atreus discovered a golden lamb which he gave to his wife, Aerope, to hide from the goddess. She gave it to her lover, Thyestes (also Atreus' brother), who then convinced Atreus to agree that whoever had the lamb should be king. Thyestes produced the lamb and claimed the throne.
Atreus retook the throne using advice he received from Hermes. Thyestes agreed to give the kingdom back when the sun moved backwards in the sky, a feat that Zeus accomplished. Atreus retook the throne and banished Thyestes.
Atreus then learned of Thyestes' and Aerope's adultery and plotted revenge. He killed Thyestes' sons and cooked them, save their hands and feet. He served Thyestes his own sons and then taunted him with their hands and feet. This is the source of modern phrase: Thyestian Feast, or one at which human flesh is served.
An oracle then advised Thyestes that, if he had a son with his own daughter (Pelopia), that son would kill Atreus. Thyestes did so and the son, Aegisthus, did kill Atreus. However, when Aegisthus was first born, he was abandoned by his mother, ashamed of her incestuous act. A shepherd found the infant Aegisthus and gave him to Atreus, who raised him as his own son. Only as he entered adulthood did Thyestes reveal the truth to Aegisthus, that he was both father and uncle to the boy. Aegisthus then killed Atreus.
Aegisthus and Thyestes ruled over Mycenae jointly, exiling Atreus' sons, Agamemnon and Menelaus to Sparta, where King Tyndareus gave the pair their wives, his daughters, Clytemnestra and Helen. When Tyndareus died, he gave his throne to Menelaus, who then helped Agamemnon overthrow Aegisthus and Thyestes.
When Agamemnon left Mycenae for the Trojan War, Aegisthus seduced his wife, Clytemnestra, and plotted to kill her husband upon his return. They succeeded, killing Agamemnon and his new concubine, Cassandra. Eight years later, Agamemnon's son, Orestes, and his daughter, Electra, returned to Mycenae and killed both their mother, Clytemnestra, and Aegisthus.