If you don't want to read on, I'll just say the film is a slightly over-egged Hitchcock pastiche, set in the 1950s. A US Marshall investigates sinister goings-on in a high security mental hospital on an isolated island. Leonardo DiCaprio is a very good actor, and really carries it, but the whole thing isn't quite the sum of its parts.
So, DiCaprio arrives on Shutter Island to investigate the escape of a female patient. But he suspects something more sinister is going on. There are 66 patients at the hospital, but he thinks a 67th patient has been expunged from the hospital records, and hospital staff deny he ever existed. As DiCaprio investigates he starts hallucinating, believes he has been drugged, and uncovers a huge sinister conspiracy involving Nazi brain surgery experiments.
The clue to the twist is the revelation early on that the escaped patient, because she can't accept her crime, has constructed a fantasy world explaining her incarceration on Shutter Island. When we learn later that DiCaprio has lost his wife in some mysterious way, and that he was present at the liberation of Dachau, we come to suspect that he is the 67th patient, and that most of what is happening is his own paranoid fantasy. Or is it?
I think it is clear by the end that DiCaprio is delusional, but I guess the possibility is left slightly open that there is a big conspiracy with him as the victim, and that rather than being paranoid, they really are out to get him. I think this possibility is left just open enough to help us empathise with DiCaprio's confusion about what is real and who he can trust.
Where I think the film goes slightly wrong is that the final 'explanation' for events is that the benign chief psychiatrist has persuaded all the staff and patients on the Island to co-operate in a huge re-enactment of DiCaprio's fantasies, in a last ditch attempt to shock him out of them. Some of what we see is re-enactment, and some is straight hallucination. This I think is nonsensical, and weakens the whole premise of the film. Much better to have him reinterpreting the normal events of the hospital, such as group therapy and medication, as elements of his imaginary investigation, with the staff and other patients gradually cottoning on to his delusions.
So, good acting, highly polished direction of course from Scorsese, but a slightly disappointing film in the end.