In the prologue we learn that they hated each other in high school and neither attended the prom; Jenko because he couldn't keep his grades up and Schmidt because he couldn't get a girl. They become best friends once they're both in training to become cops. After one fuck-up too many on the police force they get sent back to high school to pose as students and bust a drug ring. The man giving them the self-referential marching orders in the first of the film's fun cameos is the ever dependable Nick Offerman (Parks & Recreation).
In high school (the second time around) the guys get their secret identities mixed up with disastrous funny results, essentially forcing Jenko to hang with the brains and Schmidt with the cool kids led by drug dealer Eric (smirking hot Dave Franco) and open hearted funny Molly (Brie Larson). Will Schmidt and Jenko find the supplier? Are they destined to relive all their old high school mistakes?
The best thing that can be said for 21 Jump Street, beyond the smart foundational move of refusing to take itself seriously, is that it never coasts on just one type of joke. Many lesser efforts would stop thinking beyond 'ha! they're old and don't realize what's cool anymore!' but the new movie is funny in execution, not just in concept. There are a wide variety of jokes from the slapstick, sexual, social, pop culture and self-deprecating bins. Not all of the jokes land but the ratio is pretty good. It's often totally juvenile bro humor (yes, there are a few gay jokes… though they're not mean-spirited) but there's something sweet goofy about the tone preventing it from being only another raunchy R rated comedy.
21 Jump Street has a welcome number of surprises for a comedy that is essentially entirely predictable. You know from the very first scene that the movie will end at the prom. The immature cops know how to make an entrance for the grand finale (Bring your own doves) and thankfully 21 Jump Street (:The Movie) knows just how to make an exit. It wraps up at a relatively trim 109 minutes with, what else, a dumb sequel joke.