"Stoners go on wacky adventures" is a pretty good approximation of your typical Stoner Flick
plot. This is great for stoners, who will laugh at just about anything, not so much for the rest of us who like watching movies sober. In spite of having this simple concept, Pineapple Express
does a great job of amusing non-stoners (for a time) with a simple maxim- not everything needs to be implausible in order to be funny.
Take our hero, Dale. He's twenty-five years old, has a girlfriend who's still in high school, and his job is to deliver subpoenas to people, preferably while in disguise. He's a slacker, but has the bare minimum amount of smarts to realize that in order to maintain his lifestyle he shouldn't smoke weed so much that everyone notices it. He's a pretty normal dude. So when he witnesses a murder, his reaction is not to ignore it, or smoke more weed. It's to freak out
This is where the real meat of the movie is. Dale and his dealer Saul know that they're in huge trouble, but they're not responsible enough to have any idea what the appropriate reaction is. In spasms, they end up either smoking weed, running away, or trying to get help. All of these things succeed, sort of, in the sense that they don't get caught. But as our heroes are stoned, they achieve little besides this and their very believable incompetence provides great fuel for slapstick. Take Dale's attempt to warn his girlfriend and her parents that drug dealers might be coming after them- which goes so badly that even when he realizes
how much he's screwing it up he only succeeds in making the situation worse.
Unfortunately the last third of the movie loses this charm by taking away competence from the scary drug dealers and inexplicably giving it to the heroes. This results in a final extended action sequence that comes off as silly and forced, largely because up until that point it was pretty clearly implied that this movie was taking place in the real world, not one with stoned ninja drug dealers where any idiot can pick up a gun and turn into a Bad Ass
Overall recommendation- watch the movie until Dale finishes calling his girlfriend from a phone booth
. After that, it's all downhill.