Imagine asking Robert De Niro for his daughter's hand in marriage. (Shudder!) Now imagine he's an ex-CIA agent who keeps a lie detector in his basement. (Eek!) Now imagine you're Ben Stiller.

Now you see where "Meet the Parents" gets all its best laughs.

A middling comedy-of-the-uncomfortable escapade in which casting is the key, "Parents" stars Stiller as Greg Focker, a very nervous boyfriend spending a nightmare weekend at the childhood home of the girl he loves (Teri Polo).

Plagued by Murphy's Law while trying desperately to buddy up to her folks, Greg's missing luggage containing the engagement ring is the least of his problems (that is, until the wrong bag is delivered later, packed with S&M paraphernalia). He can't seem to get a laugh out of Mom (Blythe Danner) and Dad to save his life. His little white lies -- a domino effect begun when his girlfriend whispers "Oh, yeah, and we're not living together," as they arrive -- keep coming back to bite him in the backside. And before long the slapstick kicks in: A spilled urn of grandma's remains, a sorry substitute for a missing housecat and an overflowing septic tank are all pivotal gags in what should be a side-splitting comedy.

But director Jay Roach (the "Austin Powers" movies) seems to struggle with the tone of such moments, choking the chuckles out of several scenes by turning the anxiety up a little too high.

Stiller deadpans ulcer-inducing apprehension better than anyone in comedy right now, but he takes it over the top in "Parents," forgetting we're supposed to be laughing at him, not panicking with him.

When he and De Niro are playing off each other, this gag-fest is at its best -- but mostly because De Niro has a hilarious arsenal of innocuously threatening glances he keeps feeding the camera at a steady clip. He catches Stiller poking around the polygraph machine in his concealed workshop and says "Go ahead, try it on!" like a suspiciously encouraging carnival barker.

Stealing scenes from both of them is the ingeniously idiosyncratic Owen Wilson, fresh from doing the same thing to Jackie Chan in "Shanghai Noon." He plays Polo's impossibly perfect ex-boyfriend -- a dot-com millionaire and jack-of-all-trades who is tight with Pop, owns a rustic mansion he built himself and still harbors a jones for this one that got away.

From time to time a comedy bit breaks free of the movie's too-strained-to-laugh-out-loud climate. After their animosity has become a sport, Stiller and De Niro are a riot snarling at each other in a red light-green light drag-racing scene reminiscent of the sprint home between Matthew Broderick and Jennifer Gray in "Ferris Bueller's Day Off."

But more often than not, the movie's tension gets in the way of its giggles, depending too much on its stars' comedy instincts and over-extended running gags like a play on words on Greg's last name, which is beaten within an inch of its life through the course of the picture.

Some scenes might have you in stitches. But too many others will make you squirm. In six months, at home, where you can crash on your couch, control the fast-forward and chow on pizza during the plodding, cumbersome moments, "Meet the Parents" would probably be worth a $4 rental fee. But by missing it in the theater, you're not missing much.