'Shutter Island' is a psychological thriller that follows U.S. Marshals Teddy Daniels (Leonardo DiCaprio) and Chuck Aule (Mark Ruffalo) who are investigating the disappearance of a murderess from a hospital for the criminally insane on Shutter Island. As their investigation deepens, they uncover terrifying truths and question everything they know.
Teddy Daniels investigates a hospital. The hospital, gently, is investigating him back. Scorsese plants the visual clues so deep in the production design that the film functions on rewatch as a different genre.
Scorsese deliberately mismatched cuts, dialogue, glasses-of-water continuity to nudge the viewer subconsciously. On rewatch the entire production design is the diagnosis. By the time DiCaprio asks the final question the audience has known the answer for forty minutes.
The protagonist is revealed to have murdered his wife.