Last year, Mary-Louise Parker and Denis Arndt sparked enchanting, offbeat brilliance in Manhattan Theatre Club’s brief Off-Broadway run of “Heisenberg,” the 80-minute maybe-love story that the company commissioned from British playwright Simon Stephens.

The actors are back, even more nuanced and riveting, at MTC’s Broadway venue, subtly peeling layers off a vastly improbable yet profoundly believable relationship. Director Mark Brokaw’s simple, impeccably observed production has little more than a couple of metal tables and chairs with which to burrow into fine-tuned character studies. Again, the audience sits on opposing sides of a runway stage, a meaningful way to keep us close enough to see many sides of the same shifting perspectives of the truth.

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The less one knows in advance, I think, the better. In part, this has something to do with the title, a curveball about the German physicist’s uncertainty principle, often interpreted as a theory about the limitations of observation.

But, really, we need not dwell on any of that. Stephens, best known here for his adaptation of “The Curious Incident of the Dog in the Night-Time,” clearly wants to throw us off balance as delicately and brutally as he does his characters.

At first, the setup seems oddly familiar. In 1990, Parker portrayed a gorgeous, seemingly ditsy bride whose life changed after she was kissed by a very old stranger in Craig Lucas’ “Prelude to a Kiss.” Here again, she plays a gorgeous, possibly flaky woman, but, this time she’s the one who transforms reality when she impulsively kisses the back of an older man’s neck at a London train station.

Who is this woman with the dangling limbs, the floppy summer top and the way of tossing off colorful and adventurous lies — if they are lies — with an aggressive, almost feral physicality? Is she a stalker, a scammer or just the kind of babbler who gets magnetized by the vacuum created by quiet, shy people?

Arndt is at least as surprising and deeply human as Alex, a self-contained butcher from Ireland who slowly reveals unaffected shards of a poetic soul. In just a few ingratiating, ultimately devastating scenes, Stephens and his actors carve out mismatched, lonely grown-ups who are as confused and fascinated by their history and future as we are.