When Anne sees Stephen mired in marital and intellectual doldrums, she encourages him to have an affair with their beautiful friend Diana (Mikaela Izquierdo). Stephen is persuaded, Diana is willing, and everything’s just dandy until it isn’t.

“Yours Unfaithfully” is both a daring play and a highly conventional one. Under the polished direction of Jonathan Bank, and in the hands of a fine team of designers, its arguments remain provocative, while its structure feels familiar, its tone decorous. Maybe that only makes it more unusual. It’s a bit like a sex farce with real sorrow instead of slammed doors, and something like a drawing room comedy with moral conundrums peeking out beneath the cushions. It is often very funny; it is also very nearly a tragedy.

Ultimately, the play’s insistence on the sanctity of open marriage, a stance that apparently reflected Mr. Malleson’s own beliefs and practices, isn’t all that persuasive. If the central claim, that to “live effectively” you must walk the line between “a great slope of complacence on one side” and “rather a mess-up of promiscuity” on the other, sounds reasonably plausible in the moment, that is a credit to the dapper Mr. von Essen. Does the road to moral enlightenment and matrimonial contentment absolutely lead into the beds of selected others? Is there really no other way? Separate vacations, maybe?

But what is extraordinary about Mr. Malleson is his ability to create characters who are capable of feeling several things at once, or who don’t really know what they’re feeling at all. Both Stephen and Anne seem genuinely surprised that their hearts and minds aren’t as orderly as they had believed. (Ms. Gray is especially adroit at rendering these intricate emotional shadings.)