The movie takes place in the 1950s. DiCaprio is Teddy Daniels, a federal marshal, sent out to Shutter Island, an inescapable island facility for the criminally insane, off the coast of Boston. A new partner (Mark Ruffalo) goes with him as they've been tasked to find out how an inmate, a woman who drowned her three children, could have disappeared from her locked room. There is literally no way she could have escaped and yet she is gone. Things get weirder and more intense as Daniels starts to learn about government conspiracies and illegal human experimentation, and starts having flashbacks to his time in the army, liberating Dachau in WWII.
I'm not a huge DiCaprio fan but he does a great job here, arcing from capable to defensive to slowly unraveling. I guessed the twist in general terms but the movie stayed interesting and makes me want to read the Dennis Lehane book from which it was adapted.