There is no reason for “21 Jump Street” to be good.

Its source material is a ’80s television show that is only memorable for making a young actor named Johnny Depp a teen heartthrob. These were obviously the days before the eye shadow-drenched trifecta of Depp, Tim Burton and Helena Bonham Carter had combined their powers.

Plus, the history of movies based on popular television series is questionable at best, with a few successes (the “Mission: Impossible” series) and far more flops (“Bewitched,” “Land of the Lost,” “The Dukes of Hazzard,” the list goes on).

On paper, “21 Jump Street” should be abysmal; on screen, it ends up being one of the funniest movies to come out in quite some time.

The film follows Schmidt and Jenko, played by Jonah Hill and Channing Tatum, respectively, two buddy cops who help each other through police academy, only to end up as bike cops patrolling a small pond and enforcing duck-feeding policies.

After their first big bust goes awry, they’re put into a program in which the two will pose as the oldest high school students in America and try to find the supplier of a new brand of drug that looks like a communion wafer of death.

The attachment to the original series, which was essentially “Law & Order” targeted at teens, is in name only. There are a few nice cameos thrown in for fans of the original show, including Captain Jack Sparrow himself.

Outside of those appearances, however, “21 Jump Street” is more interested in making fun of its TV-to-film adaptation roots. An early conversation comments that Schmidt and Jenko’s undercover assignment is an old ’80s project being dug up due to a lack of original ideas and creativity. Writer Michael Bacall, who also co-wrote the equally energetic “Scott Pilgrim vs. the World,” is clearly having fun with the movie’s inherently low expectations, and it’s all the better for it.

The script’s cleverness doesn’t stop at breaking the fourth wall. The concept of grown-ups getting to redo their high school experiences is nothing new, but the way “21 Jump Street” takes high school stereotypes and movie clichés and flips them on their head makes it all feel fresh and reinvigorated. Tatum, who expects his massive muscles, stupidity and one-strap backpack technique to make him the king of the school, is horrified to discover that the popular kids are actually intelligent, eco-friendly and tolerant.

While we’re on the topic of Channing Tatum, it may blow some audiences’ minds to see him actually being funny and enjoyable on screen. The actor has polarized viewers across the nation: Half of the crowd thinks he’s an awful actor, and the other half thinks he’s really handsome but still an awful actor. His reoccurring appearances in romantic fluff like “Dear John” and “The Vow” hasn’t helped his cause.

“21 Jump Street” seems to have unlocked some of the charisma that audiences and casting directors have been looking for. It’s no surprise that Jonah Hill is funny, but Tatum is the real find, playing his role with a comedic reckless abandon. During the film’s funniest set piece, a chaotic drug-fueled romp through a school day, Tatum steals the show, scampering around the building with a childish glee and running through a gong while yelling something about jazz musician Miles Davis.

Tatum has always been mocked for just being a self-serious and talentless Hollywood beefcake, so it’s nice to see the actor so goofily committed to making fun of that image.

During its first half, “21 Jump Street” rides the high of its cast and script’s energy to surprisingly hilarious results. The second half, while still fun and lively, becomes a bit more predictable in terms of its story and characters.

Plus, the second half features most of the action part of this action comedy, and due to the blend of the two genres, the comedy suffers. It’s generally a lot harder to find things funny when people are getting shot and blood is flying; just ask the second half of “Pineapple Express.”

It’s that gleefully goofy first hour of “21 Jump Street,” however, that sticks with the audience. It’s hard to come down from that comedic high, but what a high it is.