The disappearance of an inmate at a psychiatric facility on an island necessitates an investigation by US Marshals Edward “Ted” Daniels (DiCaprio) and Chuck Aule (Mark Ruffalo), who make their way across the choppy waters under a grim, grey-blue sky, and soon find themselves cloistered with the facility’s director Dr. Cawley (Ben Kingsley) who, in laying down the laws of the island, appears to be a hurdle rather than a helping hand.
The Marshals conduct interviews and walk the beat around the facility in the hope of happening upon a clue. They find nothing. Amid this, Daniels confesses to Aule that he has an ulterior motive for coming to Shutter Island: to find Andrew Laeddis, an arsonist whose actions led to the death of Daniels’ wife and who was subsequently committed to Shutter Island. The mystery thickens even more when the missing inmate resurfaces, and then Daniels breaks into Ward C, declared out-of-bounds by Cawley. An inmate named George Noyce recognises him and alerts him about experiments being conducted upon inmates in the lighthouse, while also advising caution, and a perpetual distrust of everyone, including Aule.
Daniels and Aule make for the lighthouse, but the latter disappears and Daniels winds up in a cave housing a woman who claims to be the missing inmate who was earlier a member of the medical staff at the facility. She claims to have been committed when she discovered the truth about the experiments. She warns Daniels that Cawley and Dr. Naehring (Max von Sydow) will try to have him committed too, and this possibility crops up immediately upon Daniels’ return to the main facility, when Cawley insists he arrived on Shutter Island by himself.