SPOILER ALERT! If anyone hasn't seen they film, look away now!
Apparently about a bloke so in denial about his Missus killing their kids that he was institutionalised. He's completey lost his mind and makes up a fantasy to help him cope. In order to get him to come to terms with it the docs played along with this fantasy hoping he'll get it out of his system. At the end he tries to convince the docs he's better but he's not really.
Or http://screenrant.com/shutter-island-spoilers-discussion-vic-46052/