A stoner movie from the Judd Apatow syndicate was not a promising prospect to anyone who found “Knocked Up” and “Superbad” entertaining but tediously and even sloppily made. But in a typically canny move, the brand-name producer handed “Pineapple Express” to director David Gordon Green, whose reputation as an independent poet of the South and 1970s idyll is just as precisely defined and worshipped. It’s largely thanks to Mr. Green’s chops, as well as blissful work by James Franco, that “Pineapple Express” rolls along winningly for quite some time despite its excesses.

More an on-the-run caper than an indulgent mess of joint-gazing, the film throws together subpoena server Dale Denton (Seth Rogen) and his overly friendly pot dealer Saul Silver (Mr. Franco). After a normal day spoiling other people’s days and toking up, Dale witnesses a drug-related murder by Saul’s supplier (Gary Cole). Connected by the exclusive provenance of Saul’s high-grade weed, the two flee (though they don’t seem to make it far out of town). Meanwhile, a hapless middleman named Red (Danny McBride) rats them out, setting fussy hit men and a corrupt cop (Rosie Perez) on their trail.

None of this is a recipe for success at first glance, but sensibilities and tendencies sync up in ways that leave a happily dazed smile on one’s face. As Dale and Saul fall in and out with each other and the plot takes its course, Mr. Franco is sunnily likeable, offsetting the diminishing charms of Mr. Rogen (his fellow alum from Mr. Apatow’s TV show “Freaks and Geeks”). Behind the camera, Mr. Green taps another side of his beloved decade — drive-in ramble instead of Robert Altman or Terrence Malick — to give car chases and B.S. sessions a lived momentum beyond the neurotic patter of the script (by Mr. Rogen and Evan Goldberg).

The movie has its signature Apatow elements, namely the “top-this” competition of guys saying stupid stuff and the supposed slacker lifestyles, with their unpleasant odor of male fantasy and payback (25-year-old Dale has a high-school-age girlfriend). But the best comedy in “Pineapple Express” comes from the crazy logic of the chase and its absurd interludes, whether a hilarious gag with a broken windshield or simply Mr. Franco’s ingenuous grin (not the insipidly wolfish one from the poster) and cutely hurt feelings.

Some of the more slapstick moments have the tumbling, unrestrained rush of an anonymous silent short in which people pour into a room and start beating someone over the head. A prolonged three-way, in the form of a rumpus-room battle royale involving Dale, Saul, and Red, has already been justly ballyhooed. That energy can misfire, too, as when Dale sneaks back for a forgotten dinner appointment with his underage girlfriend’s parents that leads to Dad going unconvincingly postal.

“Pineapple Express” is not as cartoonish as the more bona fide pothead yarns of the “Harold and Kumar” franchise (except for a pot-origin-myth opening set in 1937). But that leaves the ramp-up of its interminable drug-warehouse shootouts in a unpleasant position. Not the first Apatow movie to run long (was “Knocked Up” really almost 130 minutes?), the movie’s buzz has fizzled by the time Dale is gunning down drug thugs and flaunting a torn ear. The finale doesn’t work as average-guy adventure or ludicrous Tarantino-esque melodrama.

Of course, the perennial odd-man-out remains the woman in these so-called “bromances,” and with Mr. Apatow teaming up with Adam Sandler for his next project, he doesn’t look set to change that tune. But “Pineapple Express” is, hopefully, a boon for the specific parties concerned. Foremost, Mr. Green gets cash and mainstream credit in the bank, even if it’s via Mr. Apatow’s template. Mr. Franco gets out of the glowering ghetto of being Spider-Man’s nemesis and overshadows Mr. Rogen, whose talk-show-host, slovenly-wiseacre shtick could go down the road of Chevy Chase in its failure to hide a faintly contemptuous smugness. At its best, “Pineapple Express” is a good old August movie, coasting down the final stretch of the summer with an affable vibe.