Nora Gavin remains haunted by a cold case that nearly cost her sanity five years ago: her sister Tríona's brutal murder. After failing to bring the killer to justice, Nora fled to Ireland, throwing herself into her work and taking the first tentative steps in a new relationship with Irish archaeologist Cormac Maguire. She's driven home by unwelcome news: Tríona's husband—and the prime suspect in her murder—is about to remarry. Nora is determined to succeed this time, even if it means confronting unsettling secrets. As she digs ever closer to the truth, the killer zeroes in on Tríona's young daughter, Elizabeth.

Back in Ireland, Cormac Maguire heads north to visit his ailing father, and hears the tale of a local woman who vanished a hundred years ago. Was she a seal-maiden who returned to the sea, or was some more sinister force at work in her disappearance?

Caught up in parallel mysteries, Cormac and Nora wrestle with identity, suspicion, truth and falsehood, and of course the biggest riddle of all—will they have a chance at a life together, or will tragedies of the past continue to keep them apart?