Teddy Daniels is a US Marshall and he goes to remote Shutter ISland to investigate an escaped prisoner/patient. Shutter Island is a facility where the violently criminally insane are treated with experimental methods. He along with his partner Chuck, find a lot of secrets in the locked doors of the island as a hurricane strikes. And Teddy finds that very little is as it seems 
 This is a very good book (and a very good movie adaptation IMO) and I leave it not sure what to believe.