The novel begins with newborn Hephaestus (the titular god beneath the sea) cast from Mount Olympus by his mother Hera. He is raised in a grotto by Thetis and Eurynome and the two goddesses tell him various Greek creation myths. The novel continues with myths of the Olympians and the age of gods and mortals, and concludes with Hephaestus returning to Olympus, having been cast down for a second time after reproaching Zeus.