The year is young, but it's hard to imagine theatrical matters getting much better in 1986 than "Sally and Marsha," an appealing comedy about an unlikely sisterhood, acted to a fare-thee-well at the Round House Theatre.

This is one of those rare instances in which an author (Sybille Pearson), a director (Gillian Drake) and two fine actresses (Gina Franz and Kathy Yarman) are so perfectly attuned that you can't really tell who's contributing what to the evening. Everything just seems right and natural, and as a result, it's immensely satisfying.

You might say that Pearson is spinning a feminist version of "The Odd Couple" (something, incidentally, that Neil Simon himself attempted with unfortunate results last season), but this is no joke-a-minute juggernaut. Funny as "Sally and Marsha" is, its humor is rooted in the hearts and minds of two housewives, who seemingly share nothing more than contiguous apartments in an Upper West Side apartment building in New York.

Pearson wants to show us how, through trials and misunderstandings, coffee klatches and morning confessionals, these two mismates become best mates. If you count friendship as one of the enduring values of life, I don't see how you can help but respond to the bonding of Sally and Marsha.

The play is, admittedly, short on plot. Subtitling it "scenes from a friendship" would not be out of order. But if, as the late director Alan Schneider maintained, dramatic action consists of "a change of relationship," then "Sally and Marsha" is full to overflowing. Its chief sin -- a certain slackness -- is more than redeemed by a superlative cast that finds truth even in the occasional easy quip.

At first it is only the perky, not to say pesky, insistence of Sally, a cheerful country mouse from South Dakota, that keeps this union from collapsing from sheer incompatibility. Marsha is the quintessential New York City mouse -- suspicious, sarcastic and prey to deep depressions, especially each time her mother comes to call. When, in a neighborly gesture, Sally offers a piece of homemade pie, Marsha thinks she's being asked to buy it. But the misunderstandings run both ways. While Sally is a whiz at whipping up those pies and "South Dakota cheddar cheese puff balls," she hasn't the vaguest idea what a bagel is. Midwest innocence is hurtling up against urban weariness.

More to the point, Sally is determined to raise the world's spirits with her corn-fed smile, pep talks and group cheerleading routines, while Marsha has a deep predilection for dwelling in the dumps. Schematic as this may seem, under Drake's probing direction the characters quickly acquire the fullness and complexity of real human beings -- Sally's chipper facade revealing its cracks, and Marsha's nihilism its strengths. Although each will ultimately benefit from the other, "Sally and Marsha" is not a simple fable about a mutual improvement society. What Pearson is celebrating is the miracle of companionship that momentarily makes the world a less terrifying and lonely place.

There are some ticklish moments, when, for example, the subject comes around to sex, and Marsha, unfulfilled by her husband, swallows her pride and asks the pregnant Sally to describe an orgasm. The two actresses bring an extraordinary delicacy of emotion to the scene that starts out in understandable discomfit and ends up capturing a rare and lovely intimacy. That intimacy, I think, is the play's real achievement and why it leaves you feeling so good.

Sally and Marsha squabble and sulk and carry on like schoolgirls. They spring jokes on each other, trade insults and alternately deride and defend their offstage husbands. But they also learn how to hold one another -- naturally and spontaneously. Solace runs all through this play.

For the Round House, it makes three in a row. I know we're not supposed to be keeping count. But after "Fool for Love" and "The Man Who Killed the Buddha," the quality of this production would argue that a renaissance is under way in the Silver Spring playhouse this season. Jane Williams' set -- Sally's disheveled apartment and the drab hall leading to it -- is first-rate. So are Susan Munson's lighting and Rosemary Pardee-Holz's costumes.

As for the actresses, they have struck such a rapport that "Sally and Marsha" does not seem acted so much as lived. As Sally, the kind of person who says "Have a good day" and means it, Yarman looks not unlike Bernadette Peters, who created the role off-Broadway in 1982. An endearing bundle of positive vibes, Yarman has also found a way to keep the character from being an insufferable simp: In the back of her mind, this Sally knows she isn't too bright. There's just a hitch of hesitation each time she tosses back her head and chuckles, as if she were waiting for outside authorization to laugh.

Franz, on the other hand, is dark, wry and handsome -- the ideal counterpoint to so much sunshine. Veiling rudeness with a subtle blend of skepticism and eyebrow arching sophistication, she lets us know early on that Marsha's urban veneer is a form of self-protection. But it is folly to separate these exquisitely meshed performances. Enhancing one another with love, understanding and generosity, the actresses themselves are emblematic of the play, which is, after all, about the coming together of opposites.

In "Sally and Marsha," one plus one makes One. Sometimes that's more than two.

Sally and Marsha, by Sybille Pearson. Directed by Gillian Drake. Set, Jane Williams; costumnes, Rosemary Pardee-Holz; lighting, Susan Munson. With Gina Franz and Kathy Yarman. At the Round House Theatre through Feb. 9.