August 2012, three fishermen discover a most unusual cache of adult sex toys, abandoned in a reservoir just south of Dublin, Ireland. When the items are linked to the discovery of a decomposed body in a forest seventeen miles away, the find takes on a more sinister dimension. The victim is Elaine O'Hara, a young woman with a troubled past. To identify the killer, police uncover an extraordinary trail, leading down a path of coercion, abuse, and extraordinary sex practices.

Police begin the extensive investigation into Elaine's murder by uncovering an obsessive and bizarre relationship. Disturbing text messages reveal a psychopath grooming his trusting victim under the guise of sadomasochistic practice and shockingly demonstrate the bloodlust obsessions of the killer and the level to which his victim was willing to acquiesce. But unlike a killer with mental illness or some criminal motivation, the chief suspect is the opposite of what police expect.

The investigation, covering three police stations, thousands of hours of video, hundreds of interviews, and an online world involving domination, submission, and torture, eventually identifies the manipulative killer. A man hiding his evil fantasies behind a reputable and unassuming mask. What follows is the longest and, arguably, highest-profile trial in Irish criminal history, revealing the methods used to track the killer and his extraordinary motivation for his depraved crime.