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Deeper and darker, curiouser and curiouser goes Jeffrey Sweet’s “Immoral Imperatives,” taking us, and the playwright, into a territory that’s risky to explore.

As in “Bluff” and “Flyovers,” two of his earlier plays, Sweet here is examining the sexual and marital ties that bind, and he is setting up his sober and moving conclusions with a casual, jokey opening.

The scene, at Victory Gardens Theater, is the Florida Keys, where Hank and Teri, a retired New England college professor and his wife, have come to live and to enjoy the company of his old pal Dale, owner of a charter fishing boat. Unfortunately, Dale’s boat has been hijacked, so Hank, out of kindness (and secret motives that become clear later), invites Dale to move in with him and Teri.

This Dale does, somewhat reluctantly, and before long, to his surprise, he is drawn into a domestic situation that he never could have imagined. Only when Liz, a trampy love from his past, shows up with her own marital woes and sexual needs is he able to extricate himself from an increasingly tangled and gloomy prospect.

Telling this story, Sweet jumps between past and present, dense philosophy and airy frivolity, first-person and third-person narration. He gets things off to a jerky start, with a rough bar conversation, followed by a windy debate on aspects of American prudery, and isn’t able to keep his balance all the time.

Just when the play begins to look and sound like an improbable bit of True Confessions pulp, however, Sweet, the play’s four actors and director Calvin MacLean throw in a killer scene that takes the story to higher level of emotion. In particular, there are a few minutes of terse, intimate talk on sex and death between Dennis Zacek’s Hank and Tim Grimm’s Dale that’s a gem of writing, acting and direction.

Zacek, a director returning to the stage as an actor, is especially fine in delineating Hank, a wry, smart, courtly educator with a deliberate gait who carries within him a hidden burden of grief and a generous layer of kindness. Linda Reiter, as his former love Liz, makes only a brief appearance in the first act, but when she comes on for her visit in the second half, chewing her gum nervously and defiantly until she breaks down in tears at the misery of her existence, she turns her trash into gold.

Kristine Thatcher, as Teri, has the play’s most difficult role, and — perhaps it is my problem, but more likely the fault lies in Sweet’s script — she is never quite convincing, despite all her acting skills, as a combination of devoted wife and sexually frustrated, sleep-around sexpot.

To the actors’ credit, and through MacLean’s direction, “Immoral Imperatives” stays on a steady course, even when it seems likely to go overboard. You can blame Sweet for the play’s unsteadiness. But you can also marvel at the simple wisdom and understanding he puts into his best scenes. A discourse by Hank on the Disney-built town of Celebration, Fla., is graced with droll insights into Mickey Mouse nostalgia; and, more important, when Hank and Teri fall into each other’s arms at play’s end, their union is eerily, powerfully moving.

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“Immoral Imperatives” continues at the Victory Gardens Theater, 2257 N. Lincoln Ave., through Oct. 28. 773-871-3000