It’s always disheartening to watch a movie that starts with so much promise, but loses just about all of its steam by the end. “Secret Window” is a psychological thriller based on the novel by Stephen King, that does precisely that. It begins with intrigue and builds tension and suspense throughout, but severely falters at the end by essentially reducing itself to just another generic thriller. During a scene late in the film, the central character says, “It’s all about the ending; it’s the most important part,” and while I might argue that the end may not be the most important part of a good story, this movie certainly proves that it can almost ruin one.

Fresh off the success of his Oscar-nominated performance as Captain Jack Sparrow in last year’s “Pirates of the Caribbean,” Johnny Depp plays Mort Rainey, a reclusive writer who, when not crouched over his computer, is usually either asleep or asking his dog for suggestions. He is in the middle of a divorce from his wife (Villanova grad Maria Bello), who he found in bed with another man in the first scene. On top of that, he is suffering from a bad case of writer’s block, which is preventing him from writing anything more than a sentence, regardless of how long he sits at his computer.

Then one day, a man named John Shooter (John Turturro) shows up at his cabin claiming that Mort stole his story. Mort initially thinks the man is lying until he reads Shooter’s story and realizes that it’s almost identical to his short story, “Secret Window.” Though Mort denies the hostile claims that he stole Shooter’s story, Shooter remains confrontational and informs Mort that he’s got three days to prove that his story was published before Shooter’s was written. Mort dismisses the man, but Shooter starts turning up everywhere and making more threats against Mort. Soon the situation spirals out of control, as Mort’s dog is murdered and his wife’s house is burnt down.

The story then kicks into high gear, taking all of the undertones of a horror story that were present all along and intensifying them. Writer-director David Koepp is no stranger to suspense or thrills, having penned the scripts for “Jurassic Park,” “Mission: Impossible” and “Panic Room,” and having written and directed the chiller, “Stir of Echoes.” He is arguably the most successful screenwriter in Hollywood (as he also has a little movie called “Spiderman” on his résumé). His skill as both a writer and director comes through in “Secret Window,” since it is very taut and well-made, building upon its conflict throughout the film to a bone-chilling effect.

As for the performances, Depp proves that he is one of the more versatile actors working today. As opposed to “Pirates,” Depp brings a level of subtlety to his role as a lonely writer. He makes the character much more interesting through little details, rather than gleefully going way over-the-top as in “Pirates.”

His performance is ultimately what holds the film together. Another equally important and interesting performance comes from Turturro, whose character is often scary. Turturro brings a unique sense of menace to the character of Shooter, providing the film with a lingering element of paranoia in his threatening demeanor.

All is well with “Secret Window” until its ending. The issue with the end is not so much that it’s bad, because it’s not, but rather that it doesn’t have the same spark as the rest of the film. Koepp went along to build and maintain a specific atmosphere and dark nature to this story, and the end is essentially a betrayal of that.

It almost feels like a different movie, and it’s not nearly as interesting as before, even though we feel it should be, considering how pivotal it is. The ending works without being insulting, but it’s disappointing; it is basically a transformation of the film from something rather interesting into a something we’ve seen a hundred times before. It simply doesn’t fit with the rest of the film.

Nevertheless, despite all its last act problems, “Secret Window” is a stylish, effective and at times, bone chilling movie that boasts a couple of really good performances and an on-the-whole involving story. That’s more that can be said of the movie from last spring based on a Stephen King story, “Dreamcatcher,” which was interesting for about 20 minutes before it got ridiculous. “Secret Window” never gets quite so ludicrous, and it also maintains a level of interest almost until the end, making it one of the better films to be based on the work of Stephen King in a while.