When Dionia arrives on the water world of Melusine she meets Hyle, the leader of a group of human colonists inhabiting an atoll city. The Melusines have adapted to an amphibious existence, but what has caused their genome to alter so quickly? And what is the secret of their symbiotic relationship with the mysterious Polyp? Hyle's people are involved in a territorial war with the merrows, a race of fierce amphibious reptiles. It is the time of the inundation, when the conjunction of the planet's two moons triggers the merrows' reproductive cycle and they rise from the ocean depths to spawn. The merrows are determined to reclaim their ancient breeding grounds from the colonists. The mysterious Shell has been lost on the ocean floor and the atoll city is defenceless without it. Drawn into the conflict by her feelings for Hyle, Dionia is plunged into a series of perilous adventures when she undertakes the quest to rescue the Shell and return it to the city. Dionia joins forces with Hressa, a female merrow. Hressa aids Dionia on her quest, and helps her to understand that the merrows are not monsters but an intelligent species with their own cultural heritage. Dionia is hiding her own secret, for she possesses mental abilities that are prohibited on her own world. On Melusine these abilities are a precious gift for they will enable her to control the Shell. But the Shell already has a Keeper - the jealous, tormented Katira; and violence erupts in a final dramatic confrontation between the two women. The Shell has a unique solution to end the war between the two species - a solution that has startling implications for humanity's future. But will Dionia manage to survive her own personal transformation?