Lucas Vega is obsessed with the death of Candace Swain, who left a sorority party one night and never came back. Her body was found after two weeks, but the case has grown cold. Three years later while interning at the medical examiner's, Lucas discovers new information, but the police are not interested.

Detective Kara Quinn, on leave from the LAPD, is on an early morning jog in her hometown of Liberty Lake when she comes upon the body of a young nurse. The manner of death shows a pattern of highly controlled rage. Meanwhile in DC, FBI Special Agent Mathias Costa is staffing his newly minted Mobile Response Team. Word reaches Matt that the Liberty Lake murder fits the profile of the compulsive Triple Killer. It will be the first case for the MRT. This time, they have a chance to stop this zealous, if elusive, killer before he strikes again.

Forensic genealogist Rowan Healy has made a name for herself by helping investigators trace the family trees of violent criminals who have eluded justice for years. But the pressure of police cases left her burned out, and she’s shifted her focus to helping adoptees find their biological parents.

Grace Holloway keeps to herself. Since narrowly escaping death at the hands of the man who kidnapped her, she’s thrown herself into the small inn she runs in Rock Harbor, Maine. It’s quiet, quaint and, in the off-season, completely isolated—the perfect place for Grace to keep her own secrets. But Grace isn’t the only one with something to hide, and Rock Harbor isn’t just a sleepy vacation town. Someone is taking young women—girls who look an awful lot like Grace did when she was kidnapped so many years ago.

Traumatised by the death of a child in her care, top London paediatrician Claire seeks refuge on remote Selkie Island in the wilds of the Irish Sea with her husband Daniel and their baby daughter Kitty. Hidden away in Daniel’s old family home, Claire hopes to be as cut off from her guilt as she is from the mainland, but can she find peace in a place so full of its own secrets? Twenty years earlier, Daniel’s parents left the island in a boat and never made it to the mainland.