Thetis was one of the Nereids. Zeus desired her, but she rejected his advances. The goddess Themis then revealed that Thetis was fated to bear a son who was mightier than his father; fearing for his dominion, Zeus gave Thetis as bride to a mortal, Peleus, and all the gods attended the wedding.

Thetis bore one son, Achilles, whom she tried unsuccessfully to make immortal. In one version of the story, she anointed the infant's body with ambrosia and then placed it upon a fire in order to burn away the mortal parts; when she was interrupted by the child's horrified father, she deserted their household in a rage. In a later version, she dipped the child in the river Styx holding him by the heel; all the parts that the river touched became invulnerable, but the heel remained dry. Achilles was later killed in the Trojan war.