The great “National Lampoon’s Vacation” inspired a so-so Christmas followup (lots of dead weight in that movie), plus a couple other sequels that all parties have agreed to simply never mention again. The newest sequel, called “Vacation,” is the first to focus on the next generation of Griswolds, and the first to really recapture the loose, anarchic, slightly dark spirit of the 1983 original.

Young Rusty Griswold is grown up now, played by Ed Helms in the image of his father: a nice guy, clumsy, hapless, often clueless, but usually well-meaning. Overhearing that his wife, Debbie (Christina Applegate), and their boys — 16-ish James (Skyler Gisondo) and 13-ish Kevin (Steele Stebbins) — are tired of their usual Memorial Day vacation at a lakeside cabin, Rusty decides on a whim to take everyone on a cross-country drive to Walley World, just as his own father did back in the day. (The film gets a few meta-references out of its system — “Will this ‘Vacation’ be as good as the first one?” “This ‘Vacation’ will stand on its own,” etc. — before moving on.)

The story of a family that drives 2,500 miles when they could have flown is almost inherently going to be discursive and rambling. “Vacation” is filled with — one might even say consists entirely of — tangents, vignettes, and side trips that are, strictly speaking, unnecessary. For example, a stop at Debbie’s college alma mater just as her old sorority is throwing a party doesn’t advance the plot, and what little pertinent information it reveals about Debbie’s character could have been established more efficiently elsewhere.

But none of that matters if the sequence is funny, which this one is. Like most of the movie, it’s gloriously rude, with humor that ranges from the cheerfully offensive to the borderline surreal. (The sorority girls repeat legends they’ve heard about Debbie, culminating in “I heard that if people bop you on the head, gold coins come out of your butt.”) The cast is game: Helms as earnest dad who embarrasses everyone; Applegate as supportive but frustrated matriarch; Gisondo as the dweeby, bookish older brother; Stebbins as a grinning bully of a younger brother. The gleeful manner in which the little bastard torments his big brother made me laugh long and loud. All four of them get a turn at being hilariously bitter or passive-aggressive, but never to the point where you feel like they hate each other. They’re a fractious family, but not an atypical one.

Not every diversion between home and Walley World is of equal value. The ongoing gags stemming from Rusty’s disastrous choice of rental cars (“the Honda of Albania”) wear thin quickly, and the brief appearance by Chevy Chase and Beverly D’Angelo as Grandpa and Grandma Griswold adds nothing but nostalgia. (The inadvertent dip in a pool of raw sewage, prominently featured in all the film’s advertising, is likewise a dud, though there are funny moments in the process of getting there.) But writer/director duo John Francis Daley and Jonathan M. Goldstein maintain a high success rate, earning laughs both cheap and clever from a visit to Rusty’s sister (Leslie Mann) and her impossibly hunky husband (Chris Hemsworth), a trip to the Grand Canyon (with Charlie Day as rafting guide), an illicit encounter at the Four Corners, and so on. There’s very little filler in these 99 minutes. For a movie built on meandering, it’s surprisingly well-oiled and smooth-running.

B (1 hr., 39 min.; R, pervasive harsh profanity and a lot of sexual dialogue.)