At school, Hannah and her three best friends, Laura, Jenny and Annabel, were inseparable. Known first as The Inseparables, then later as Martin's Mermaids after they had become the stars of the school swimming team. It was a close and enduring bond during their school years. So when Laura calls out of the blue with the idea of holding a reunion (it has been eighteen years!), Hannah is excited at the prospect of seeing how the years have treated her contemporaries who she has lost touch with. But a reunion that should have been an occasion for nostalgic reminiscence and light-hearted catching-up, turns into a nightmare, when one of them is found dead, drowned in her own swimming pool.