Rating:

It may mine well-trodden territory, but the new comedy “The Hangover” is a delight — a hilarious romp through Las Vegas that remains uproariously funny for its entire running time. Unlike most comedies of its kind, it never runs out of steam, nor does it turn into a sappy romance in the third act.

Directed by Todd Phillips, best known for helming “Old School,” the film is simultaneously a comedy and a mystery: Four friends — jerk Phil (“Wedding Crashers” villain Bradley Cooper), meek dentist Stu (“The Office’s” Ed Helms), creepy nerd Alan (Zach Galifianakis), and groom Doug (Justin Bartha) — head for Vegas for Doug’s bachelor party.

After a drunken night, they wake up, minus Doug, and spend the rest of the movie figuring out exactly what happened, and trying to find Doug so he doesn’t miss his own wedding.

Throughout the adventure, the men come across various animals, a baby, a stripper, angry Asian gangsters, and even Mike Tyson, who contributes an entertaining cameo. The screenplay by Jon Lucas and Scott Moore keeps the laughs coming until the very end, including a closing-credits montage that pays off the movie’s mysteries perfectly.

The film is outrageous, but never mean-spirited, which is where it departs from 1998’s “Very Bad Things,” another bachelor-party-in-Vegas-gone-awry comedy that went right past comedy straight into nihilism. “The Hangover” has more in common with “The Pineapple Incident,” a first-season episode of “How I Met Your Mother” in which characters also tried, after the fact, to piece together a drunken night.

At any rate, it’s the best bachelor party movie since, well, “Bachelor Party.” This is a comedy that’s actually well-served by a less-starry cast, as it’s probably a better movie than it would’ve been if, say, Will Ferrell, Vince Vaughn and Jack Black had played the three main parts. Cooper lays on the smarm as always, but his persona is just right for the role.

Helms gets away from playing his “Office” character, and makes the most out of what could have been a one-dimensional part. (Rachael Harris, as his nightmarish girlfriend, channels Leslie Mann in “Knocked Up,” had the character been a little less nice.) But the real revelation is Galifianakis, best known for a short-lived cable series and the excellent “Between Two Ferns” segment on the Funny or Die Web site. He plays a Jack Black-like part but does it much better than Black probably would have, playing it a little understated but very, very creepy.

It’s the most impressive breakout performance in a comedy since Black himself in “High Fidelity.”

The film also makes room for some memorable supporting performances. Apatow mainstay Ken Jeong, fresh off memorable turns in “Knocked Up” and “Role Models,” steals a couple of scenes as an effeminate gangster.

A couple of ex-“Daily Show” characters, Matt Walsh and Rob Riggle, also turn up in funny parts, while Heather Graham — in her first role of consequence in many, many years — is a welcome site as the stripper.

Phillips, who made “Old School” seven years ago and followed it up with the “Starsky and Hutch” remake and “School For Scoundrels,” has made his best film ever, one that’s the early favorite for comedy king of this summer — a film certain to be watched and re-watched on cable for years.