Remember what the world was like before Judd Apatow brought back the raunchy, R-rated comedy? Nah, me neither. Oh wait -- I do! Hollywood comedies, for a good long while, were PG-13 at their naughtiest, aimed at as broad a demographic as possible, and they weren't very funny. It was a tough time to be a grown-up with a sense of humor.

But The 40-Year-Old Virgin and Knocked Up changed all that, and now we're blessed with a wide variety of adult comedies! By "wide variety," I naturally mean "stories about grown men suffering from protracted adolescence." Pineapple Express, Role Models, The Hangover, I Love You, Man ... all very funny. And all, when you think about it, essentially about the same bunch of guys.

Hot Tub Time Machine is rated R, it's very funny, and it once again features the same group of characters, although they're rendered even more sketchily than in any of the above-mentioned films. John Cusack plays Adam, the likable anchor/ostensible main character. He's just broken up with his live-in girlfriend, who we only experience via phone message and Post-Its demanding her share of the household goods. Adam and and his friend Nick (Craig Robinson) reunite at the hospital following a possible suicide attempt involving their loose-cannon pal Lou (Rob Corddry), and they decide to visit the old stomping grounds from their youth, an upstate ski resort called Kodiak Valley. Adam's nephew, Jacob (Clark Duke) comes along, as well.

There is a hot tub. They get in it. And then it's 1986. Advised by a mysterious hot-tub repairman (Chevy Chase) that they must do everything exactly like they did when they were at the resort some 20 years earlier, Adam is charged with breaking up with his girlfriend, Nick needs to perform on stage, and Lou must get the crap knocked out of him by some asshat ski patrol guys. And so, they proceed to do just that, with mixed results.

It would have been prudent, perhaps, to write "spoiler alert" before the above synopsis because, really, that's all there is to the movie. Like The Hangover, which this film seems to admire, what follows is anecdotal, disjointed, and slapdash, more a series of gags inspired by the central premise than a real plot. Yes, it's often very, very funny. Not all of it is, but enough that it makes for an enjoyably stupid experience. Transported without warning to '86 via a hilariously well-crafted time travel sequence, the four understandably freak out. They run down the rules of time travel as learned from the Terminator flicks; Jacob, in danger of disappearing altogether if the boys don't follow the rules, proclaims himself an expert on the subject because "I write 'Stargate' fan fiction; I know what I'm talking about!"

Duke, Corddry and Robinson are reliably funny, no matter what they do, even when a couple of outdated, homophobic scenarios are trotted out. There are jokes about oversized cellphones, leg warmers and Michael Jackson. The soundtrack pulls out some of the most egregious examples of 80's schlock-pop to create a kitsch soundtrack. Through it all, Cusack, working again with director Steve Pink (Pink co-wrote Grosse Pointe Blank and High Fidelity), simply looks tired. It's charitable to say that this is the funniest Cusack comedy in a long time -- but his last comedy was 2005's Must Love Dogs, and he never breaks a sweat in Hot Tub Time Machine, mostly going through the motions and allowing his co-stars to do all the heavy lifting.

Coming very close to stealing the movie is Crispin Glover, as a creepy one-armed bellhop. His surly, antagonistic demeanor when they arrive at the hotel -- followed by his demand for a tip -- is over-the-top and deliriously funny. But it gets even better when he reappears in 1986, chipper, cheerful and possessing all his limbs, then suffers from a series of near-arm-severing accidents.

Women are, of course, mostly set dressing, serving as floozies, bimbos, wives and ex-girlfriends. Lizzy Caplan brings a welcome smart female note to her few scenes as a music journalist who catches Cusack's eye, but otherwise it's a lad's adventure all the way. Given the scattershot nature of the film (and the outtakes/alternate lines that pepper the advertising yet never made it into the movie) there'll probably be a lot of funny extra material on the DVD release. Meanwhile, for those of us old enough to get references to Red Dawn, 80's hip hop, Back to the Future, and Sixteen Candles, there's a fair amount of genuine retro fun to be had here.