Scorsese uses every tool at his disposal to keep the audience off-balance:

[Spoilers] What Really Happened in Shutter Island? : r/TrueFilm

The narrative begins as a classic mystery: two marshals arrive at Ashecliffe Hospital on a remote island to investigate the disappearance of a patient, Rachel Solando. However, Scorsese uses the island’s isolated setting and an oppressive, storm-battered atmosphere to mirror the protagonist's fractured internal state. Through stunning cinematography by Robert Richardson—characterized by muted tones and stark, claustrophobic lighting—the environment becomes a literal manifestation of Teddy’s psychological prison. Trauma as a Narrative Engine