Good Half Gone delves into the haunting tale of Iris Walsh and her quest for the truth behind her twin sister Piper's disappearance. As a teenager, Iris narrowly escaped the same chilling fate that befell her sister—abduction and a presumed life of trafficking. The investigation, hindered by scant evidence, grew cold and was eventually abandoned.

Now an adult, Iris is driven by a single goal: to unearth proof of what happened to Piper. With the police unwilling to reopen the case, Iris takes matters into her own hands. She secures an internship at Shoal Island Hospital, a remote facility for the criminally insane. It's a place shrouded in secrecy, where the shadows hold more than just darkness.

Iris's determination to uncover the truth leads her to discover that the hospital's sinister undercurrents run deeper than anyone imagined. The patients might be confined, but they're not the only ones under surveillance. As Iris delves into the hospital's mysteries, she realizes that the true danger may be lurking much closer than she thought.