When we see Martin Scorsese's name attached to a film, our expectations are naturally going to be loftier than usual, sometimes unreasonably so. After all, he's the greatest working American director, and even in his 60s he hasn't compromised the tough, savvy sensibilities that made his early masterpieces so memorable.

That's a rarity, if you think about it: Most of the great filmmakers from the 1970s who are still active, like Francis Ford Coppola and Woody Allen, have seriously lost touch with their audiences, while Scorsese continues to make one excellent picture after another-"Gangs of New York," "The Aviator," "The Departed," "No Direction Home." He's a national treasure, really, a filmmaker who knows how to extend his talent across genres and generations without ever compromising his personal style.

Scorsese's newest is "Shutter Island," a pulpy, stylish locked-room mystery based on a novel by Dennis Lehane, author of "Mystic River" and "Gone Baby Gone." It's a good movie, well-executed and mostly involving, but, considering the great director's legacy, it's seriously underwhelming. It's one of those thrillers in which nothing is as it seems, and no one-not even the protagonist-can really be trusted.

If you're like me, watching a movie like this can be maddening, because you're reading into every little clue and nuance in hopes that you'll be able to solve the riddle before the characters do, desperately trying to differentiate between legitimate evidence and red herring. By the end, though, "Shutter Island" has so many loose ends to tie up that it takes forever (a good twenty minutes if I estimate correctly) to explain itself, and the outcome really isn't worth all the effort we've put into it.

This seems like an open-and-shut case of a promising film that doesn't quite live up to its credentials, but I have to wonder: If someone as great as Martin Scorsese hadn't directed this movie, if it had been a springtime sleeper or a late summer thriller, would I have been as disappointed as I was? Is it fair to raise your standards while watching a Scorsese picture, but lower them again while watching any other director's work? After all, this is the guy who made "Taxi Driver," "Raging Bull" and "Goodfellas," and we've been spoiled with the expectation that he's gonna hit it outta the park every time.

That's not to say that "Shutter Island" doesn't have impressive merits: It's beautifully photographed, the supporting cast is a who's who of great character actors, the set-up is intriguing, and some of the early scenes crackle with tension. That would be high praise for most films, but then most films aren't directed by Martin Scorsese. This is one of his weakest efforts, and even though I liked a lot of it, I can't help but wish it was better.

It'll be difficult to detail the plot here without giving away too much, since every little detail plays into the Big Reveal at the end. It begins with the striking image of a ferry pulling into a fog-choked harbor: Two U.S. marshals, Teddy Daniels (Leonardo DiCaprio) and his new partner Chuck Aule (Mark Ruffalo), are investigating the disappearance of a patient from Ashecliffe, a maximum-security mental institution that houses murderous psychopaths.

The missing woman, Rachel Solando (Emily Mortimer), has been living in a fantasy world perpetuated by her doctors and psychiatrists-she doesn't realize that she drowned her three children in a fit of insanity and believes the hospital staff to be milkmen, mailmen and acquaintances from her old neighborhood. But how could she have escaped? She clearly left without her shoes, there were guards and orderlies right outside her room all night, and the facility itself is surrounded by jagged cliffs and rough waters.

But the plot thickens. Actually, it solidifies. More and more mysteries pile atop one another-Is the head shrink (Ben Kingsley) slipping Teddy drugs? Is the brash German doctor (Max van Sydow) conducting inhumane Nazi experiments in the lighthouse? What lurks in the shadowy, drippy bowels of Ward C? What are Teddy's real motivations for coming to Shutter Island, and is it related to the horrible tragedies from his past?-as each new scene clouds our perception of the previous one. The plot moves along at a breathless pace, and as the truth becomes murkier, as suspicions shift from character to character, as Teddy's sanity continues to slip out from under him, it's kind of fun to be swept up in the crazy momentum of things.

And then, in its last thirty minutes, "Shutter Island" comes to a dead, screeching halt, as the screenplay sits us down and patiently, laboriously explains everything that came before. ("Hey, remember all that nutty stuff that happened in the last two hours? Well, here's what it all meant!") It completely foils the forward thrust of a picture that, up until this point, had been almost operatic in its intensity; it's a jarring about-face that sort of reminds me of that unnecessary, long-winded medical diagnosis at the end of Alfred Hitchcock's "Psycho." And as for the explanation itself, it's a real downer, and one that I'm sure half of the audience will have guessed before it's revealed.

I would still have all the same problems with "Shutter Island" had someone lower on the filmmaking totem pole been behind the camera. I would still have given it the same rating, and recommended it with the same caveat that the story doesn't sustain itself all the way through. The only difference, I guess, is that lingering sense of disappointment, the feeling that we've been watching a great director slumming it for a couple of hours.

So, as far as psychological thrillers go, "Shutter Island" definitely strives harder than most. As far as Scorsese pictures go, however, it's mostly forgettable. I recommend it for its style and craft, but don't go expecting magic to happen.