Answer
Shutter Island is a novel by Dennis Lehane which was adapted into a film by Martin Scorsese. The novel tells the story of Teddy Daniels, a U.S. Marshal who is investigating the disappearance of a patient from Ashecliffe Hospital, a federal mental institution located on Shutter Island. As the investigation progresses, Teddy begins to question his own sanity and the reality of what is happening around him.