Between “The Simpsons Movie” and the conclusion to the Harry Potter series, the pop culture firmament has been groaning under the weight of anticipation in the past week. With the blockbusting young wizards out of the way, the stage is set for the fraught debut of Springfield on the big screen. It’s appropriate that the animated images of Homer and company now cast shadows in their cinematic incarnation, because “The Simpsons” has certainly cast a long one, thanks to its nearly 20-year run, protean endurance, and wide-ranging cultural impact.

The problem is how to step out of that shadow, and “The Simpsons Movie” must tread a tricky path between satisfying an encyclopedically equipped fan base and a general audience. Even among devotees, there are several camps deeply dug in at different milestones along the 18-season spectrum. And the filmmakers implicitly stake a claim by drawing the staff of 11 writers from earlier eras of the show.

All of which is worth restating, even after weeks of anticipatory press coverage, because “The Simpsons Movie” often plays like a veteran comic holding himself back. Good but not great, the movie does flex its comedy muscles dazzlingly in ways recalling the show’s golden years. Yet the show’s snap ultimately seems muffled by the feature length, a yawning expanse compared to the fruitful compression of the average episode.

Still, a muffled “Simpsons” outclasses almost any comedy in theaters today, as the film’s first half demonstrates almost offhandedly. Typically, the story gets under way thanks to Homer’s lovably oblivious selfishness. After adopting a toothsome pig, destined for slaughter, Homer precipitates an environmental disaster by dumping the ineffectual beast’s waste into the town’s lake. Almost as bad, he lets Bart get arrested for some naked skateboarding that arose out of a friendly father-son dare contest.

The Environmental Protection Agency takes swift action on Springfield and does the only logical thing to do in a widescreen cartoon: enclose the town in a giant glass dome. No one can leave, and the angry mob that brews means Homer must flee with his family. He chooses Alaska as his refuge, but Marge’s patience — “the ability to overlook everything you do” — is worn perilously thin. Meanwhile, the ruthless EPA head, Russ Cargill, pushes the nation’s president (President Schwarzenegger, that is) into plans wipe Springfield and its threat off the map.

The basic premise of temporarily broken family relations is familiar from the show, and several bits of the movie are grounded in past episodes. Flanders is again eager to adopt Bart, who’s deeply disappointed by Homer, while Lisa again has a crush on a fellow environmental activist (a musical chap with an Irish brogue). And Russ Cargill’s Cheney-esque power grab and overpowering homeland security efforts are one of many typically topical nods, evenhandedly paired with Lisa’s unsuccessful lecture entitled “An Irritating Truth.”

Both gestures feel more like merely adequate echoes than consistently funny references, and the satire is already a little stale. (The best jabs come at the expense of Disney, including a perverse redeployment of friendly forest animals.) But the writers repeatedly must save the film with zingers to electrify a plot that never really achieves a pleasing loop. As it toggles between Homer’s separated family and the Springfield disaster, “The Simpsons Movie” reminds the viewer what a clockwork device one good episode could be.

The 30-minute show could contain multitudes, after all, while the movie, in its weaker second half, threatens to fall into the rhythms of a single adventure template as parodied in the trailer. Homer’s roundabout route to saving his town, marriage, and family doesn’t display typical shortfalls of lazy leaps from television to film, but more a nagging misjudgment in adapting the elastic television show. Marge’s disappointment and Homer’s blithe irresponsibility are doled out somewhat abruptly and sloppily, without finding the delicate balance of credible, earned sentiment that the show achieved.

Nobody is going to “The Simpsons Movie” for the sentiment, surely, but it was always a reliable complement to the movie’s irony and absurdity. And when the one-liners fail, as in Homer’s interminable visit with an Inuit medicine woman, the movie can get a little quiet. Nor does “The Simpsons Movie” push the potential of the roomier visuals very far, with the filmmakers erring on the side of focus rather than density. But an exhilarating opening sequence with Springfield’s own cartoon stars, Itchy and Scratchy, certainly has an enviably manic pace.

The cast — Dan Castellaneta, Julie Kavner, Nancy Cartwright, Yeardley Smith, Hank Azaria, Harry Shearer, et al — rise to the task of voicing the town’s well-represented armada of faces, joined most notably by Albert Brooks as Cargill and briefly by Tom Hanks as a bland celebrity spokesman. If the movie doesn’t indulge as many tangents and priceless throwaways as the show, we at least get to hear Bart slurring after a liquor binge.

It’s perhaps inevitable that “The Simpsons Movie” should be a bit disappointing, after looming for so long as an obligatory phase in cultural touchstone’s life cycle. And experiences will doubtlessly vary according to tastes for past vintages of the show (I’d rate it somewhere right after Season 8, with high notes from earlier on) and the layers of generational loyalties, reflected by the critic who brought his son to the screening. But if “The Simpsons Movie” can make just one more person enjoy a potbellied man with permanent five o’clock shadow making an ass of himself on a grand scale, then perhaps it’s done all it needs to do.