Metamorphoses Book 8: Achelous and the Nymphs
Theseus and his men were on their way home to Athens when they came to a swollen river. Achelous, god of that river, invited them into his cave. He told them how the islands in the distance had once been water nymphs, but he'd transformed them to islands because they forgot to include him in a feast they had. An island a little farther away had once been Perimele, his love. Her father had found out about their affair and thrown her into the sea, so Neptune turned her into an island for Achelous' sake.
Theseus and his men were all impressed by the stories with the exception of one who made light of the power of the gods. An old man told the next story as a warning to the young man who scoffed at divinity.