Neil LaBute has spent his career in the company of men, and generally these have not been nice men. The guys who have made their way out of Mr. LaBute’s imagination, onto the page, then onto the stage or the screen, have not tended to be the sort that another guy would want his sister or daughter to end up with.

These men have been creeps, mostly. Lowlifes, lots of them: physically violent, nasty, unfaithful, bigoted, vaguely criminal, cowardly, homophobic, shallow, reflexively dishonest, pathologically susceptible to peer pressure. Not the full laundry list in one guy, necessarily, but Mr. LaBute’s view of his fellow men has not been overly charitable. Even that almost-sweet guy who defies his rotten friends and falls in love with the overweight librarian in “Fat Pig” is only almost sweet. He dumps her. The rotten friends win.

But now along comes “reasons to be pretty,” and suddenly evolution isn’t just a theory anymore. With Mr. LaBute’s hilarious and hopeful new play, making its world premiere at MCC Theater, the bad-boy playwright with the oppressively misanthropic worldview is changing before our eyes.

Greg (Thomas Sadoski) is the hero of “reasons to be pretty,” and “hero,” for once, is the proper word. Never mind the fact that when we meet him, he is in the midst of a ferocious fight with his longtime girlfriend, Steph (Alison Pill), whose wounded rage he has provoked with an overheard act of cruelty: In a beer-soaked conversation with a friend, he called her ugly.

Because lying is Greg’s default setting, and because he is neither brave nor stupid, he denies having said it. He backpedals, he deflects, he attempts to charm, he tosses in a red herring about Wonder Woman, but we know he’s guilty. And when Steph leaves him, he knows he deserves it.

He doesn’t understand that right away, though. He has an unwitting instructor in his cocky, bullying co-worker and pal, Kent (Pablo Schreiber), a slimeball whose knockout wife, Carly (Piper Perabo), is way out of his league. It was Carly, Steph’s friend, who reported Greg’s betrayal to her, and it is Kent’s repellent treatment of Carly that finally forces Greg to realize that’s not the kind of guy he wants to spend his life being.

Also helpful in pushing him in that direction is the spectacularly merciless, highly comical, two-page declaration of his physical shortcomings that Steph reads out loud to him, post-breakup, in the food court of the local mall.

“I could feel all that and still love you,” she tells him when she’s finished, though she’s so haunted by his calling her ugly that she can’t sleep at night. Words said in anger can be taken back, Mr. LaBute suggests, but words said calmly in an unguarded moment can inflict harm that can’t be undone.

The irony is that Greg ought to have known better. He’s a smart guy who’s set the bar too low for himself: an autodidact who reads Poe and Swift on his breaks at the warehouse where he works the third shift.

Mr. Sadoski is the magnetic center of director Terry Kinney’s cast, which deftly navigates the emotional terrain of Mr. LaBute’s dark comic landscape. In Mr. Sadoski’s poignant, beautifully nuanced portrayal, we see not only why Steph loves Greg but why Greg hates himself.

Ms. Pill — who, for the record, is perfectly attractive, even radiant in later scenes — is not at her most convincing when Steph is blindly furious, and she never quite knows what to do with her arms, but her reading of Steph’s list, with its mixture of anger, offhandedness, and surprise at herself, could be a master class.

Ms. Perabo gives a somewhat brittle performance as the prim, officious Carly, the slightest and least rewarding of the play’s four roles. But Mr. Schreiber is fascinatingly odious as the dull-eyed, adolescent Kent, by far the ugliest person onstage. “You’re disgusting,” Greg tells his friend. And it’s true. Yet Mr. Schreiber lets us see the fear and insecurity that feed his to-the-bone rottenness, the desperation behind the too-quick smile.

Kent is the kind of guy Mr. LaBute has understood for a long time, the kind of guy who may be aware of the wrongness of his behavior but does nothing to change it. What sets “reasons to be pretty” apart is that Mr. LaBute is telling a story of redemption, and Greg is his own redeemer.

What Mr. LaBute comes up with by way of a moral — and, make no mistake, it is exactly that — will not be news to most people: that it’s important to be nice, and it doesn’t take much effort. But in the world of Mr. LaBute’s plays, that message is nothing short of revolutionary.

Until July 5 (121 Christopher St., between Bleecker and Hudson streets, 212-279-4200).