Acis was born of divine blood on both sides, though his power was quiet and local. His father was Faunus, the Roman god of the woods and fields, identified by the Greeks with Pan, the goat-legged deity of flocks and shepherds who played the syrinx in the mountain glades. His mother was Symaethis, a Naiad nymph of the river Symaethus, the great Sicilian waterway that flowed from the Nebrodes Mountains around Mount Etna to the sea. So the river was already in him before any of it began, a lineage of water and woodland that made him at home among the thyme-scented slopes of Etna. He was sixteen when the story found him, old enough to tend his flocks but young enough that his beauty still carried the softness of boyhood. He pastured his sheep on the lower flanks of the mountain, where the grass smelled of warm dust and wild herbs, and where the sea lay close enough to hear.

Galatea loved him. She was a Nereid, one of the fifty daughters of Nereus, the old man of the sea, and his wife Doris, the Oceanid of the ocean's bounty. Homer and Hesiod both name her among the Nereids, the sea nymphs of Greek tradition. She was milk-white and sea-bright, and she chose the Sicilian shepherd over every suitor on the coast. The two of them met where the shore met the wooded slope, where shade fell across cool stone, and she would lay her head against his shoulder and listen to him talk. That was all. The myth makes nothing more of it than that. A boy and a nymph in the shade of the rocks above the sea.

But they were not alone. Polyphemus, the Cyclops, had fixed his single eye upon Galatea long before. He was the giant son of Poseidon, the god of the sea and earthquakes, and he was the same one-eyed shepherd who would later be blinded by Odysseus on his voyage home from Troy. For now, he still had his eye, and it was entirely given to Galatea. The Roman poet Ovid spends a long passage on his vanity. The Cyclops combed his bristling hair with a rake. He cut his shaggy beard with a reaping hook. He sat on a wedge-shaped cliff above the water with a panpipe of a hundred reeds and sang to the indifferent sea, listing every gift he could offer: orchards, apples, goats, cheese, even a pair of bear cubs for her to play with. The seer Telemus had once warned him that Odysseus would one day put out his eye, and the giant had laughed it off, joking that another had already robbed him of his sight, so completely had love overcome him.

After his song, he discovered the two lovers in the shade below him. His bellow split the air like thunder, and Mount Etna itself shuddered at the sound. Galatea, hearing it, fled into the sea. Acis turned to run, but he was not quick enough. Polyphemus tore a corner off Mount Etna itself, a piece of the mountain large enough to be a hill in its own right, and threw it. Only a fragment of the stone touched Acis, and that was the entire fragment that needed to. The boulder buried him where he stood, and crimson blood ran out from beneath the rock.

Galatea did not abandon him. She called upon her own divine power and upon the ancestral water-right that Acis carried through his river-blooded mother, and the answer came. The stone split. A tall green reed pushed up from the crack. Water leapt out of the hollow, and from the fissure rose the boy himself, transformed. He stood waist-deep in his own new spring with woven rushes around his temples and fresh horns curling out among them, the standard iconography of a Greek river god. He was larger than he had been, and his face was a deep blue, the colour of deep water, yet it was still Acis. He had become the river, and the river was him, and from that day the water carried his name down the mountain to the sea.

The story is thinly attested in ancient literature. Ovid's Metamorphoses preserves the earliest surviving detailed account of Acis's death and transformation. Earlier Greek poetry had already explored Polyphemus's love for Galatea, including the Cyclops or Galatea of Philoxenus of Cythera and the pastoral poems of Theocritus. These earlier works provide a background for the jealous Cyclops, but they do not by themselves establish an earlier surviving version of Acis's complete story. Whether Ovid reshaped lost material or developed the episode in a new direction remains uncertain. The association with Sicily and the landscape around Etna is central to the tale.

In the centuries after Ovid, the tale became a favourite of Renaissance painters and Baroque composers. Nicolas Poussin painted the lovers hiding from the giant. Claude Lorrain set them in golden landscapes. Handel made the tale into the pastoral work Acis and Galatea, first performed in 1718 and later revised, with a libretto associated with John Gay and other poets. The myth had travelled from a Sicilian stream to the courts of Europe, a reminder that even the most local and fragile love can become immortal if the water remembers.