The glittery opening night audience assembled for cocktails in front of Waterfront Playhouse on Wednesday evening entered the theater to encounter Michael Boyer’s most alluring set yet.

Like in the best scenes of “Citizen Kane,” it has a ceiling, the kind with soft hidden lighting one would expect to find in a Park Avenue penthouse. The fixtures and fittings, courtesy of Chris Elwell of Royal Furniture, are in every shade of plush — oyster bisque, chamois, taupe and slate gray.

Ah, let us live here for the next hour or two! Such an invitation … and such a riotous outcome.

Paul Rudnick’s “Regrets Only” is farce at its best, so beautifully performed that the lunacy becomes absolutely believable. Directed at a lightning pace by Danny Weathers, the reality deepens even as the vanities of the protagonists grow madder.

The trouble begins when Jack McCullough, played by George Murphy at perfect pitch, is invited by the president of the United States to help craft a constitutional amendment on marriage. As it happens (this is a Paul Rudnick play), Jack’s best friend — and, most importantly, his wife’s best friend — is gay. The amendment, naturally, is not.

Murphy manages to melt into the part of Jack, hapless foil to wife and daughter and, ultimately, to all the gay people of New York and Washington. Yet he is truly tough and he wins out at last. So Murphy gets to play the story of his life for the run of this wonderful play.

The best friend and Jack’s unlikely skating partner is clothing designer Hank Hadley (“Fashionable is about fashion. Style is about you”), played with laconic and intelligent ease by Mike Mulligan. Hank is one of Rudnick’s most realistic creations and one of Mulligan’s finest renditions. His high-fashion gowns can be found all over the place (“especially on prom night,” says one character) but he himself is harder to grasp. It’s a subtle point, but without Mulligan as Hank, this production of “Regrets Only” would be altogether another play.

So would it be without Karen Grant, who is about to be the talk of the town as the play proceeds through its season (it runs to Jan. 10). She is Jack’s wife Tibby and for most of the play she is clad in a couple of off-the-shoulder outfits, Hank’s of course (in actuality by Carmen Rodriguez), that deserve and demand an appropriate exquisiteness in the wearer. Grant exceeds such expectations like she was the star of a 1930s Hollywood comedy swooning with glorious glamor and rich raunchiness. The audience fell for her, hung on her every word, adored her mind like a dive into an empty blue swimming pool (she is both blonde and red-haired at various points). Grant, a lifelong professional entertainer, has arrived on the Key West stage. A star is born.

Tibby’s hot daughter Spencer, edgily played by Christina Olivieri, is a strange part because she evidently loves both her fiancé and Hank, but hey, as another character says, “That’s when you know it’s love — when it’s strange.” She is delightful as she swills back the water from the flower vase to swallow her Xanax. Spencer is having fun but she’s serious about it, and the same goes for Olivieri in the role.

The indomitable Robin Deck is the ideal grandmother for Spencer. She gets to enter stage left attired only in garbage bag and traffic cone (to protect her new hairdo), a look she pulls off as though it were an everyday thing. “Is it OK?” she asks. “It will be,” says Spencer. “I’m a lawyer.”

Stealing the stage every time she sets foot on it is Ronnie Goldstein as the maid, Myra. Using every known accent and some unknown, Goldstein plays this other strange part at full tilt and torques it upward from there. (Her impromptu dance during a scene change prompted applause.)

She is helped, if help were needed, by author Rudnick’s delectation for one-liners and punch lines. It’s a complicatedly constructed script, occasionally quite musical in its counterpoints and sometimes just flat-out funny.

“Have you ever been married,” the maid is asked. “A door man, perhaps?”

“Get out!”

“It could happen.”

“No, I mean it. Get out.”

Gay or straight, “Regrets Only” at the Waterfront is the riot for you. It is what they call “lol” in computer-speak, meaning laugh-out-loud. Enjoy.