There are few things more discouraging than going to see a film that you have low expectations for, being happily surprised by a better-than-expected first hour, and then having your newfound hopes cruelly dashed by a second half that quickly unravels before completely falling apart. Such is the unfortunate case with "Good Luck Chuck," a bawdy romantic comedy that is closer to what comic-turned-actor Dane Cook probably had in mind when he took on 2006's idiotic "Employee of the Month." For a long time, the film is hugely funny, winsomely paced and, dare it be said, even smart in its own crude and tasteless way. Trouble ultimately tiptoes up behind the good will and then pounces without warning, plummeting the proceedings with asinine plot developments, dumbed-down character actions, and a conclusion that is as derivative as it is insultingly dishonest.

In 1985, pre-teen Charlie's (Connor Price) love life was hexed by a peeved goth classmate (Sasha Pieterse) when he refused her advances during a game of "Seven Minutes in Heaven." Twenty-two years later, a grown Charlie (Dane Cook), now a dentist, has yet to meet that special someone. That is not to say that he doesn't get sex, though, as word spreads around town that sleeping with him unequivocally blesses each gal the destiny to find their true love with the next guy they meet. The endless one night stands fast grow tiresome—a sentiment that perplexes horny best friend Stu (Dan Fogler)—as he yearns for a real girlfriend.

Charlie is finally convinced he's met his match after a meet-cute with the terminally klutzy, cute-as-a-button Cam Wexler (Jessica Alba), a penguin handler with a heart of gold and a resistance to getting too emotionally involved with the opposite sex. Despite her best judgment, Cam and Charlie start going out and hit it off splendidly. The honeymoon's over after they have consummated their relationship, though, with a paranoid Charlie sure that the pattern will continue and Cam will be swept off her feet by the next guy she meets. His consequent attempts to win her heart for good are, to put it mildly, disastrous, placing a storm cloud over a possible future together.

Directed by Mark Helfrich, a professional film editor making his directing debut, "Good Luck Chuck" is an outrageous, at times extremely raunchy, R-rated comedy in the vein of such recent hits as "Knocked Up" and "Superbad." Although the laughs arrive at a pleasingly clip rate, spurred by pointedly shrewd dialogue (courtesy of some choice and unrepeatably foul one-liners) and perfectly-timed physical gags (courtesy of an acrobatic sex montage and Cam's accident-prone run-ins), the movie loses steam just as its conflicts thicken and its script, by Josh Stolberg, switches to autopilot.

In a seeming instant, the picture's entertainment value and, until now, sweet if overly conventional love story are botched in exchange for one deeply flawed story turn after another. Charlie, or "Chuck" as he is nicknamed, faces an unnecessary quandary that wouldn't really be a quandary at all had the character been written with a semblance of common sense. After sleeping together, he believes that Cam will meet someone else immediately afterwards that will steal her from him. This logic holds zero water, however, because all of the previous hookups he has had have been with women who were only using him as a means to meeting her soul mate. By comparison, Cam genuinely likes Charlie, and there isn't a single sign that would suggest she wouldn't be monogamous with him, especially since she states earlier that it takes her a while to allow herself to get close to anyone.

Film-destroying error #2 is in the supremely foolish way in which Charlie deals with his insecurity: he turns into a creepy psychopath who stalks and figuratively smothers Cam to the point where she is scared of him. These actions, forced upon the narrative by a screenplay that treats the audience like imbeciles, destroys the likable relationship that has been built between these two characters. Even if their initial getting-to-know-you dates are marked by archaic cinematic chestnuts—moonlit walks on piers, a carriage ride, ice cream-eating on top of the hood of a car as airplanes take off overhead—Charlie and Cam's chemistry was pure enough to overcome it. By the end, which includes the oldest convention in the romantic comedy handbook—that's right, a last-minute dash to the airport—the viewer no longer has any rooting interest in these people getting together because it is clear that Charlie has made inalterable mistakes and Cam deserves better than him.

The cast, at least, is game. Dane Cook is the straight man of the film as Charlie, but he gives good reactions to the more colorful characters around him. Jessica Alba (2007's "Fantastic Four: Rise of the Silver Surfer") is radiant as Cam, and turns out to have quite a bit of go-getter spunk in the physical comedy arena. The jury's still out on Alba's range as an actor, but her inviting presence makes up for her emotional limitations. Dan Fogler, as randy breast-centric plastic surgeon Stu, is worlds more charismatic than he was in the lead role of 2007's miserable "Balls of Fury." Last but not least is Ellia English (2004's "Woman Thou Art Loosed"), a comically tart as well as unexpectedly touching standout as Charlie's office receptionist Reba.

When a film has as much going for it as "Good Luck Chuck" did at the onset, and the magic is flying, it is positively heartbreaking to watch it all deteriorate by way of such glaring condescension. As the helmer of the piece, and one that exhibits a good deal of promise, director Mark Helfrich doesn't play fair with his audience. He expects that we will go along with the thoroughly moronic plot even as it sacrifices respect for Charlie and the truth of the central romance. Nevertheless, there comes a point near the conclusion of "Good Luck Chuck" where a possibility for redemption presents itself. At the expense of his own happiness, Charlie makes a beautiful gesture that he knows will make Cam happy. When the screen fades to black after a scene where Charlie walks away from the zoo exhibit, the end credits should have rolled. Sure, the movie would have left the viewer on a bittersweet note, but it would have been authentic and right. By pressing forward for another ten minutes and trying to squeeze out a predictably happy denouement that only rings false, "Good Luck Chuck" signs its own death warrant.