In Shutter Island, Teddy found himself on an island which situated an asylum that a patient - Rachel Solando - had escaped from. As a detective of sorts he has been called in to investigate and find the missing patient with a man named Chuck that he had not met until he was put on the case. But Teddy had been wanting to go to the island for a while, to investigate for other reasons. First he believed that the island wasn't as much there to help as it was to experiment and second the man who killed his wife was supposedly imprisoned somewhere on it.
The more time Teddy spent on the island, the more he believed that Rachel had never existed and that the evasive doctors there really wanted to keep him stuck there so he couldn't prove his theories of experimentation to the rest of the world. The most unnerving realization he had to begin with was that he didn't know anyone on the island and therefore they could all have been working to keep him there, including Chuck who he's been trusting whole heatedly up until then. He also found that it was strangely coincidental that he'd been the one chosen to investigate on the island he'd been trying to get to and that as far as he could tell there was every possibility that Rachel Solando never existed. As the story progressed things only became more strange, he found a woman in a cave who looked like Rachel, only quite a bit older. She claimed that she'd been one of the first doctors on the island and didn't want to be a part of it once they started using methods that she was uncomfortable with and was unable to leave after seeing what she saw. Since then she'd been hiding from them in that cave. Finally she warned that every time Teddy ate, drank or smoked something on the island he was being drugged with something that would make him lose his mind and that the headache he had was one of the first signs.
Now, the end is a bit confusing and could be taken two ways. One, that Teddy was right in his inductive leap and that the doctors won. Or as I see it, two, that Teddy wasn't who he believed himself to be anymore and had actually been treated on the island for several years as a highly delusional patient. There's a lot more to the end than that but this post is already too long so I'll end it at that.
