“Our Idiot Brother” has a menage a trois, nudity, pot use and profanity. But the unfailing sweetness of Paul Rudd’s lead performance makes what could have been another raunchy and rude R-rated farce  a bracing change of pace in a summer of aggressive comedies about aggressive people, from “Bad Teachers”  to “Horrible Bosses.”

Rudd plays Ned, a “bio-dynamic farmer,” when we meet him, a soul so open-hearted and trusting that he listens to a sob story from a uniformed cop at his farmer’s market stall, gives him the pot he begs for and then stares, slack-jawed, as the jerk slaps cuffs on him.

“I’m such an idiot,” Ned mutters, and not for the first time.

He won’t get any argument from his sisters. The bossy and stressed magazine writer Miranda (Elizabeth Banks), bi-curious failing comic Natalie (Zooey Deschanel) or smothering, politically-correct mom Liz (Emily Mortimer) think he is — their word — “retarded.”

“Sap” might have been more polite. Ned’s laid-back, aimless victimhood means his girlfriend moved on while he was in jail and that she won’t even let him retrieve his beloved dog, Willie Nelson, from the farm where they lived.

He could just move back into his old room in his kind-hearted mom’s (Shirley Knight) rural Long Island home, the one with the Peter Sellers posters on the wall. But Ned tests himself and his relationships with his sisters by moving in with each one in turn, letting his guileless demeanor and his unfiltered self-expression come into conflict with their images of themselves and of him.

“If you put your trust out there” with the best intentions, is Ned’s credo, “people will rise to the occasion.”

Only they don’t. His jerk brother-in-law (Steve Coogan, the jerk du jour), his probation officer (Sterling Brown), they all act on meaner instincts and Ned is the poorer for it.

Rudd can play the innocent as well as the snarky cynic, and that serves him wonderfully here. This is a Peter Sellers tribute, a hapless hippy version of the struggling Indian actor Sellers gave us with “The Party” over 40 years ago, filtered through Randy Newman’s sarcastic ode to those not on life’s acquisitive track — “It’s Money That Matters.”

Ned is so mellow that even the guy (T.J. Miller, even mellower) who stole his girl is Ned’s soul brother in spirit, another dude he can call “Man” with affection. And here’s the worst insults Ned can summon up for the woman (Kathryn Hahn) who kicked him out and kept his dog.

“You know what? YOU KNOW WHAT? Wow.”

Even Natalie’s lesbian lawyer girlfriend (Rashida Jones) can’t teach aggression into this wise fool.

“Who’s the MAN?” she demands, summoning up Ned’s outrage.

“You are?” he wonders.

The director of “The Ex,” Jesse Peretz, can’t quite take care of all these characters and stay on message. It’s a lot to keep track of. Ned nannies his nephew (playing with the kid since his parents won’t), states the obvious about Miranda’s “best friend” neighbor (Adam Scott) and the lack of moral compass she has to have to write celeb gossip for Vanity Fair. He forces Natalie to try a little honesty in her relationships with her lesbian lawyer girlfriend and the artist (Hugh Dancy) she sometimes models for, in the nude. He makes Liz aware of the lie she’s living.

Rudd, sweetly and shaggily (he wears very long hair and beard and shorts and t-shirts throughout) holds this all together, a sort of non-conformist cliche who is, above all, comfortable in his own skin. The great joy in this adorably slight film is seeing how absolutely beguiling to men, women and children this man without guile can be.

MPAA Rating: R for sexual content including nudity, and for language throughout.

Cast: Paul Rudd, Elizabeth Banks, Zooey Deschanel, Steve Coogan, Emily Mortimer, Shirley Knight, Kathryn Hahn.

Credits: Directed by Jesse Peretz, written by David Schisgall and Evgenia Peretz. A Weinstein Co. release. Running time: 1:30