“Treatment” is the funniest, most energetic film yet to come out of the mumblecore movement, which has proven to be as incestuous as it is prolific. Its star, Joshua Leonard (whose beak nose and shaggy charm resemble Owen Wilson’s), and director/producer Steven Schardt were also involved with 2009’s “Humpday.” That movie’s co-star Mark Duplass was the co-director of 2008’s “Baghead”; Duplass’ wife, Katie Aselton, directed, wrote, and starred in last year’s “The Freebie,” which also featured Leonard. Some of the troupe—Duplass and Greta Gerwig, most notably—have paved their way into mainstream films.

Until now, these mumblecore films have retained a depressing, stilted sameness. They are small in scale, taking about a week or so to shoot in one or two settings, and hew to the mumblecore style of stammering dialogue, tight close-ups and an endlessly shaky camera, all in the service of achieving so-called realism. The stories are relatively one-note, throwing banal characters into “wouldn’t it be crazily awkward if…” situations: two straight guys get duped into shooting a gay porn movie, a couple agrees to a night of infidelity, etc. The films seem more like cloying improv class exercises than actual fleshed-out stories.

“Treatment” has a similarly jokey plot: a struggling screenwriter (Leonard) pretends to be a heroin addict in order to be admitted to a high-class rehab center, where he tries in vain to pitch his ideas to a narcissistic, A-list action film star named Gregg D (Ross Partridge). It also has moments of shoddiness; the camera goes in and out of focus, and certain small characters’ monologues seem thrown in arbitrarily.

But “Treatment” happily abandons most of the other annoying traits of mumblecore. Schardt and co-director/co-star Sean Nelson (the former singer for one-hit-wonder Harvey Danger) actually keep the camera steady for most of the movie, allowing the actors’ deadpan approach to shine through without distraction. And rather than building a whole movie around reticent people, Schardt and Nelson fill the characters with absurd, loopy charisma. As a result, “Treatment” isn’t just more amusing than this team’s previous efforts; it’s also less claustrophobic and more ambitious. That it falls apart in the end from an unwarranted shift in tone—from lighthearted satire to poignant melodrama—doesn’t detract from this significant improvement.

The first half of “Treatment” is dotted with hilarious misadventures. Leonard first attempts to recruit Partridge in a dive bar’s mens room where he promptly gets pissed on. Embarrassed, the drunken Partridge cleans Leonard off, then tells him to come to his table and discuss movie ideas. Some ten seconds later, Partridge has disappeared, and the next day, he’s admitted to Wingspan, a rehab center set up like a resort, complete with equestrian sports, a sundae bar and a hysterically gauche promotional video promising all the inhabitants perfection.

In desperation, Leonard convinces his put-upon best friend (Nelson) to con his older brother into funding the rehab’s entry fee, and then, in the film’s funniest scene, Nelson and Leonard drive around inner-city Los Angeles looking for drugs.

For a while, “Treatment” is a timely satire of the feel-good condescension employed at rehab centers (Dr. Drew, anyone?). Chris Caniglia is a comic marvel as the unctuous, jargon-spouting head doctor. There’s also plenty of sharp digs at how ridiculously easy it is for rich addicts to smuggle drugs into rehab.

The friendship between Leonard and Nelson is developed nicely. Nelson, looking like a no-nonsense collie with his plump, beady-eyed, hangdog face and Z.Z. Top beard, brings out the perfect mixture of exasperation and tenderness. And Leonard isn’t afraid to display his hapless character’s maddeningly self-centered delusions of grandeur. You like him as much as you want to smack him, and when he does finally get slapped by a caring but militant counselor (Jessica Makinson), it is no less than he deserves.

But “Treatment” fumbles when it suddenly takes a serious, overwrought turn, as Leonard actually becomes addicted to painkillers. Leonard’s gradual redemption, as he realizes that the goal of recruiting Gregg D is a futile, self-important dream, provides enough dramatic tension on its own. The drug addiction subplot seems thrown in to allow for neat little camera tricks, as well as for tacking on an all-too-familiar message about the consequences of drug experimentation.

Overall, though, “Treatment” is a promising step forward for the mumblecore pack, and it’s likely that Leonard, Schardt and crew will do even better next time.