Metis was of the Titan generation and an Oceanid, a daughter of the Titans Oceanus and Tethys. He was the first spouse of Zeus, before Hera.
Metis was also the one who gave Zeus a potion to cause Kronos to vomit out Zeus' siblings. After the war against the Titans, Zeus married Metis, but she would soon became a threat to Zeus and his indispensable aid.
"Zeus lay with Metis but immediately feared the consequences. It had been prophesied that Metis would bear extremely powerful children: the first, Athena and the second, a son more powerful than Zeus himself, who would eventually overthrow Zeus."
In order to forestall these dire consequences, Zeus tricked her into turning herself into a fly and promptly swallowed her. He was too late: Metis had already conceived a child. In time she began making a helmet and robe for her fetal daughter. The hammering as she made the helmet caused Zeus great pain, and Hephaestus hit Zeus's head at the river Triton, giving rise to Athena's birth. Athena leaped from Zeus's head, fully grown, armed, and armored, and Zeus was none the worse for the experience.