"School for Scoundrels" is breezy and genuinely funny for so long that it's a shame writer-director Todd Phillips and co-screenwriter Scot Armstrong lose their way in the third act. Like Phillips' and Armstrong's previous collaboration, 2003's "Old School," they take a promising premise and then proceed to muck it up with one-note characterizations and an outcome that is not only misguided, but also embarrassingly flat. In fact, an end credits sequence set in a supermarket with supporting player Sarah Silverman (2003's "The School of Rock") is almost a replica of an end credits scene in "Old School" with Juliette Lewis—a sure sign that all creative ambitions have long since subsided.

For the first time since his breakthrough role in 2004's "Napoleon Dynamite," actor Jon Heder proves that he might actually be more than just a flash-in-the-pan celebrity who got lucky and briefly hit the big time. Here he plays Roger, a New York City meter maid who is a virtual doormat for everyone he meets in his life. Tired of being scared all the time and letting people walk all over him, he decides to attend a confidence-building class taught by the imposing Dr. P (Billy Bob Thornton). Dr. P's methods are radical—in one hilarious montage, the students are sent a message on their beepers to immediately orchestrate a confrontation, leading one to grab a violin from the hands of a friendly-looking Asian woman in Central Park and bash it over a tree—but they actually begin to work for Roger. He reclaims his dignity against two men who had previously stolen his shoes when he tried to give them a ticket, and even scores a date with next-door neighbor Amanda (Jacinda Barrett), whom he has had a longtime crush on. Roger's upward swing is bound to hit a bump in the road, and it most certainly does when Dr. P swoops in and threatens to steal Amanda's heart. For Roger and Dr. P, this means war.

"School for Scoundrels" is loosely based on a 1960 film, unseen by me, called "School for Scoundrels or How to Win Without Actually Cheating!" Without already knowing this, one would never be able to tell that it's a remake. The modern-day "School for Scoundrels" has the sensibilities of a comedy of the 21st century, what with its broad, physical, and sometimes crude humor. The film surprisingly works more often than not in the first half, as Roger wholly commits to the instructions of Dr. P's class and, lo and behold, starts to gain confidence and come into his own. Some of the lines are real zingers in Todd Phillips' and Scot Armstrong's script—when Dr. P secretly breaks into Amanda's apartment and frames Roger as a love-crazed lunatic, a fake cop nonchalantly tells her she is sure to be killed. Furthermore, there is a dignity brought to Roger's character that keeps him endearing; even when following the sometimes unsavory guidance of Dr. P, Roger does not compromise his morals in exchange for a backbone.

When concentrating at the onset on the unorthodox class and the gentle romance between Roger and Amanda, "School for Scoundrels" is at its high point. Once Dr. P turns into an antagonist out to ruin Roger's life, the good will of the first hour slowly drifts away as the tone uncomfortably darkens. Suddenly, Amanda is turned from a respectfully written person into a cheap pawn stuck between one man whom she is made to think is psychotic and another man who cruelly manipulates her. It all finally leads to a sloppy, antiquated climax involving a mad dash to the airport and a heated showdown on the plane between Roger and Dr. P. This finale isn't as funny as it thinks it is, and is more mean-spirited than it wants to be. And then comes the epilogue, one of those desperate "what-happened-next" sequences that is not only pointless, but deals with characters who have been so underdeveloped up to this point that their fates mean nothing to the viewer.

Jon Heder is the star attraction as Roger, and carries the movie better than anticipated. He creates a character of depth even when the screenplay doesn't support this claim, and is an authentically likable presence who deserves all the happiness he strives for. The changes, internal and external, that Roger goes through during the course of the film are subtle but noticeable, and Heder does well in establishing this arc.

The rest of the cast is underutilized. Billy Bob Thornton gets top billing as Dr. P, but doesn't really have much screen time. Mostly, he is called upon to play a thinner, less humanized version of the characters he played in 2005's "The Bad News Bears" and 2003's "Bad Santa." As Amanda, Jacinda Barrett (2006's "Poseidon") is asked to be warm and sweet and pretty, and she fulfills all of these demands. The side actors are a mixed bag. Michael Clarke Duncan (2005's "The Island") displays his lighter side as Lesher, Dr. P's brute assistant. Talented comic Sarah Silverman is wasted as Becky, Amanda's bitchy roommate. And Ben Stiller (2004's "Meet the Fockers") doesn't get a single laugh as a previous student of Dr. P's who now lives a broken life with a house full of cats.

"School for Scoundrels" is a disappointing fumble of failed promises, slowing to a crawl when it should be gaining momentum in its homestretch, and introducing an unsettlingly contemptible side that is just plain ugly rather than edgy. Because Roger and Amanda are such a cute and attractive couple, it would have befit director Todd Phillips to take advantage of their chemistry and drastically cut the dreary revenge plotline that ultimately rises to the forefront. There is enough that is entertaining and, indeed, comedically amusing about "School for Scoundrels" that things aren't completely destroyed when the missteps begin. Still, it's a film of schizophrenic tonal shifts and sadly missed opportunities.