Chrissy Hart is trying to write her novels and become the next Agatha Christie sensation while raising her son, Charlie, and running a spiritual retreat at the Zennor Hotel in Cornwall. Her life is quiet, disciplined, and carefully ordered.

Then her ex sister-in-law returns.

JD Collins is not only strikingly beautiful, she is wildly successful. The travesty of it gnaws at Chrissy. People like that; everything comes easily. JD's novel The Mermaid's Chair is being filmed in nearby Zennor at the legendary Mermaid Cove, a place whispered to hide secrets in its cliffs and waters. She is dating the enigmatic director Tom Sagnier, whose fascination with the myths of the cove borders on obsession.

And then the girls begin to vanish.

Disappearances ripple across the moorlands. When Chrissy's son's girlfriend, Marie, goes missing, her quiet life is shattered. JD Collins is found dead while dating Tom Sagnier, who is accused of being a serial killer and imprisoned; until Chrissy helps free him. When she moves in with him, her old boss, private detective Ryan Kahn, delivers a chilling question: are you living with a murderer?

Adjacent to Mermaid Cove lies District 9, a top-secret US military base where no one talks about what goes on underground. Strange experiments, surveillance, and disappearances are whispered about; but some say the truth is far stranger: real mermaids, dragging the unwary into the sea, never to be seen again. Chrissy must confront what is real and what is legend, and decide how far she will go to uncover the secrets lurking beneath the waves.