• Courtesy of Gerry Goodstein
• Tyne and Tim Daly in Downstairs

Tim and Tyne Daly have built distinguished careers in television and theater, but not until Theresa Rebeck, Dorset Theatre Festival's resident playwright, wrote a script expressly for them have they appeared together onstage. The real-life brother and sister play siblings in Downstairs, a stunning drama that keeps the audience in doubt about reality until it strikes a jarring truth that transforms three fascinating characters.

Dorset launches its 40th season with the world premiere of Rebeck's play. Downstairs couldn't have a better showcase, for the production is not only technically superb but features subtle, engrossing stage acting. The Dalys bring powerful performances to the little theater in southern Vermont, and John Procaccino, in a smaller role, is equally impressive.

Tim Daly plays down-on-his-luck Teddy, who's been living in his sister's basement for a few days. Tyne Daly is Irene, his older, married sister whose husband, Gerry (Procaccino), isn't too pleased with having a guest in the house. Irene enjoyed having a baby brother, but their childhood was tough, with an alcoholic mother and a father who abandoned them.

Rootless Teddy is content in the basement, but his explanations about his employment include ideas that seem delusional. Irene has a solid middle-class life going on in the house above, keeping up with housework and accepting her husband's preference for fast food over her cooking. We seem to see a brother and sister testing who should be the parent and who the child. It appears obvious which of these two has problems and whom we can trust.

The play shatters those assumptions as both Irene and Teddy confront serious struggles. The very way each appears a little untrustworthy is the essence of their uncertain foothold in the world. People rarely lie in this play. They just can't say what they're sure of. The story gives the audience a chance to wonder if we're seeing factual events through our own eyes or episodes imagined by one of the characters.

Director Adrienne Campbell-Holt sustains Rebeck's fascinating test of what's real with staging that fiercely asserts authenticity yet always leaves room for doubt. Campbell-Holt draws nuanced performances from the three actors, supporting Rebeck's finely tuned ear for the rhythms and syntax of natural speech. Characters stammer, overlap and fumble for their thoughts. The director builds on this naturalism to make the story appear intensely private, and she supplies movement and pace to grip the audience from beginning to end.

One scene in Downstairs is so tense that a ripple of anxiety seemed to surge through the full house at Friday's opening night. When actors bring singular clarity to the work of a dazzling storyteller, theater liberates the audience to feel deep emotion.

Rebeck toys with truth throughout the storytelling, but it's no idle trick. Instead, the play demonstrates that tearing down a sense of reality can obliterate the sense of self.

All of the actors work subtly, building their characters from within. Tim Daly, whose character has mental health issues, portrays Teddy's effort to maintain coherence without indulging in a superficial and flashy version of disintegration. By showing us the character striving to cope, he builds an astonishingly complete portrait. Daly lets nervousness surge through Teddy and often curls his thumbs up inside his closed fists, figuratively hiding. Hollow-eyed one moment and happy the next, Teddy is continually fascinating as he lurches through his moods. Rebeck supplies Teddy's wit and his woes; the actor reveals his heart.

Irene travels the greatest distance in the play, and at every turn Tyne Daly quietly discloses new truths about her character. She lets Irene search hard for her observations; we see Irene's reluctance before each little revelation peeks through. Daly works from deep inside her character, letting her voice trail off and averting her eyes so her toughest statements start to dissolve even as they're delivered. She blends conviction and vulnerability in each moment. At her weakest, Irene holds some ground; at her most powerful, she can't let go of doubt.

Procaccino gives a spellbinding portrayal of a normal husband who doesn't much like the idea of a brother-in-law living in his basement. He conveys the character's power with small but intimidating gestures, making Gerry's strength an uncontested fact. As he surveys the basement, patting his belly, Gerry looks like an average American husband. Procaccino allows us to overlook him at first, but Gerry will stand out clearly in the end.

The scenic design by Narelle Sissons jumbles heaps of basement clutter around a sofa that's broken down but comforting. This basement has both the warmth of a refuge and the chill of abandonment. Sissons underscores the play's mood and meaning with a ceiling of exposed joists and ductwork. By marking the edge between the basement and the house above, the design echoes the story's questions about what's seen and unseen.

Michael Giannitti's skillful lighting matches the intensity of the play even as it evokes the utilitarian illumination in a basement, showing the sharp angles of a workshop's bulbs, the fuzzy sunlight at a basement window and the bright light from that other world at the top of the stairs.

ML Dogg's exhilarating sound design bridges the scene changes with energetic, percussive music. Charles Schoonmaker's costumes complement the show's unassuming naturalism. He uses offhand leisurewear to convey each character's sense of self-worth, from Teddy's rumpled chaos to Irene's shaky self-image to Gerry's easy indifference.

A basement is a provisional space, hidden and unfinished. Memories are much the same, shifting over time beneath the surface. In Downstairs, a basement becomes a crucible. Teddy strains to imagine an implausible future; Irene struggles to see the present clearly. And both wonder how the past has shaped them. The result is a moving, cathartic theater experience.