Can you trust what know?
This reminded me of of the films that Alfred Hitchcock delivered. He didn't need an R rating or lots of gore, instead he built atmosphere with various visuals, and implied outcomes. To do this properly does take a little time but the result is a more intense movie.
We are introduced to a US marshal Teddy Daniels who is sent to investigate the disappearance of on of the patients at mental hospital. This is set in the mid fifties and the hospital is on an island, so there can't be too many places to hid a body.
It soon becomes apparent that perhaps not all of the hospital's practices are in the best interest of the patients. There seem to cases where radical and unorthodox treatments have been used along with unethical and probably illegal practices as well.
But in order to prove all of this he needs the vary institution he is investigating to turn over their records. Assuming that management knew of all the practices going on it is no surprise that they refuse to grant him access to their medical records or treatment procedures used.
It just also happens that severe storm is closing in, this cuts off communications to the outside world. When this is coupled with people he has interviewed disappearing, and patients managing to escape out of their rooms during the storm.
It becomes increasing difficult to split truth from fiction, so much so that he begins to question the motives of his partner, and even his own memory. Maybe something you don't want to be doing in a mental health institution, you might just end up as a resident.