A sealed room. A murdered psychiatrist. A decades-old secret someone will kill to protect.

When Dr. Marcus Holloway is found dead inside his locked office—surrounded by disturbing patient files from a long-buried experimental program—Captain Kate Gazzara is pulled into her most dangerous investigation yet. Someone is hunting down the survivors of a secret psychiatric trial gone horribly wrong.

Armed with surgical precision, digital surveillance, and a motive rooted in trauma, the killer is always one step ahead. The files name 47 patients. Now they’re dying one by one.

Who is Subject 23? And how do you catch a killer who already knows you’re coming?