The "Pineapple Express" of the title is some supercharged grass, of which Saul is the sole supplier. Boys and girls, if you don't think drugs kill, just ask Dale, who witnesses a murder committed by a drug kingpin named Ted (Gary Cole) and a rogue cop (Rosie Perez). Dale, a doughy process-server, sees Ted shoot a rival drug lord, and gets so fatutzed that he drops his telltale supergrass roach - which is how Ted is able to track him back to Saul.
Saul's ever-so-polite supplier Red (Danny McBride, consistently funny) gets involved in the shenanigans too, when the kingpin's muscle (Kevin Corrigan and Craig Robertson) leans on him. Red sings like a bird, and Dale and Saul end up running for their lives - which does not stop their reefer madness at all.
As Dale observes in a rare moment of clarity, marijuana is not a performance-enhancing drug. It's loaded with comedic potential, however. There is by now a well-established (if not venerable) tradition of pothead comedies, which combine drug-induced cognitive impairment with a goofy buddy/road picture shtick; think Cheech & Chong cross-pollinated with Crosby & Hope. If you can conceive such a combination, and you think stoners and their misadventures are funny, then Pineapple Express is the movie for you. Pineapple Express might have been even funnier if it didn't devolve into a knowingly dumb, intentionally excessive shoot-'em-up at the end, when it tries to walk the fine comedic line between parodying a dopey action movie and actually becoming a dopey action movie. It certainly gets the dopey part right.
Pineapple Express is funniest when its characters are shooting the breeze instead of bullets. For the most part, it's an amiable, mellow, occasionally hilarious tale of two addlepated dudes who can't say no to drugs. Franco is particularly funny, soulful, and winsome as Saul, the lonely dope dealer who has many visitors, but few friends. Saul is as wacky as his tobacky, giving voice to all manner of dingbat theories, tuned in to a wavelength all his own.
Saul's gentle goofiness provides a nice counterpoint to Rogen's excessively excitable Dale. It's theoretically if not physically possible that Dale needs even more drugs. Despite his slacker ambitions, Dale manages to have both a job and an unlikely girlfriend (Amber Heard), a high school student who does not appreciate getting dragged into Dale's problems with the drug fiends.
Like any good road movie, the pothead road movie is more about the journey than the destination. Pineapple Express gets that mixed up for a while, but it hits the groove whenever it slows down and focuses (in an out-of-focus kinda way) on bud and blooming buddyhood.