Explain the ending of given movie.
Shutter Island
First, we watch the story of a federal marshal who arrives on the island to investigate the disappearance of a patient in a mental institution. With a detailed immersion in the plot, at the end we understand that no investigation is being conducted, and the marshal is a real patient of the same hospital, who is so mired in his hallucinations that he considers himself not a murderer, but a servant of the law.