Shutter Island opens with a seasick Leonardo DiCaprio barfing in the john aboard a ferry en route to Shutter Island, where mystery awaits. Contrast DiCaprio — shaky, sweaty and pale — with fellow U.S. Marshal Mark Ruffalo — calm, erect and pristine — and you have in a nutshell not only each character's psyche, but also the film's love affair with introductory-level symbolism and metaphor (e.g., do you think an insane asylum on a remote island is just a cool setting for a psychological thriller, or does it represent something?). However rudimentary, the juxtaposition works, and we have in the first two minutes a complete profile of each protagonist. Such is the case with Shutter Island as a whole: getting the job done by way of elementary techniques.

Martin Scorsese's adaptation of Dennis Lehane's novel arrives thoroughly soaked in a dark, threatening atmosphere that dwarfs in its shadow the film's more hackneyed plot elements. The movie builds just as much tension with its thundering, uneasy score and eerie set designs as with the series of nightmarish episodes that plunge high-strung DiCaprio and levelheadedRuffalo deeper into despair. They're summoned to Shutter Island to solve the disappearance of a patient who miraculously escaped from her locked cell. When their investigation branches out into a series of other mysteries, the only real challenge they face is discovering if there even is a mystery to begin with. In the process, the venomous island becomes as much a character as the cast of crazies (including Maxvon Sydow, Elias Koteas, Patricia Clarkson and Jackie Earle Haley, Hollywood's go-to guy for demented lunatic roles) that inhabit it. Because he plays to the environment's strengths, Scorsese builds a world in which virtually any thriller story could succeed.

But a psychological thriller such as this will more likely than not be measured almost entirely by its solution, and Shutter Island slams the brakes on a two-hour-long buildup of tension with a denouement both aggravating yet oddly satisfactory. The well-executed final scene is something special on its own, not least of all a prime example of how to conclude a labyrinthine thriller and when exactly to cut to black. Standard, overblown thriller though it may be, Shutter Island succeeds because of its restraint, its atmosphere and its careful presentation of reality and truth. It's as simple or as complex as the viewer wants it to be.