The romantic comedy-drama “Constellations” opens conventionally enough, with opposites attracting. Its two British characters are a humble and easygoing beekeeper named Roland, who lives in some remote outskirt of England, and the flamboyant Marianne, who analyzes data about quantum mechanics and theoretical physics at Cambridge University.

But Nick Payne's play diverges quickly — and then, by design, repeatedly. Theirs is not so much a case of love at first sight as it is a case of love at fifth or sixth sight, too. In a theatrical bit of "cosmic relativity," and with no "linear explanation," Roland and Marianne proceed to live and re-live the pivotal moments from their relationship any number of times. In Payne's alternate or parallel "multiverse," the slightest differentiation in their actions or re-actions can alter the entire course of their affair.

In Horizon Theatre's production, director Justin Anderson (who previously helmed "The City of Conversation" for the company) employs subtle contrasts in the lighting (by Mary Parker) and understated sound or music cues (by Rob Brooksher) to distinguish between the many reiterations of each scene. And there's a nice celestial simplicity to the set (by Isabel and Moriah Curley-Clay) — two bare benches at center stage, encircled by strands of shimmering strings.

Still, however handsomely executed it is, and however briskly it moves along during much of its 80 or so minutes, the premise gradually begins to wear thin, becoming not just decidedly repetitious but periodically redundant as well. Before it’s all over (and over and over) and done with, melodramatic complications inevitably arise for the couple, at which point the show loses a lot of its fanciful, fantastical spark.

Horizon Theatre’s “Constellations” features Bethany Irby and Enoch King. CONTRIBUTED BY BRITT ELSE
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The splendid performances Anderson elicits from Enoch King and Bethany Irby make a whole universe of difference. Both of them thrive and relish in the opportunity to work outside of their so-called “comfort zones.” (Their British accents are technically fine, if somewhat distracting. You almost wish Anderson had opted to Americanize the roles instead.)

King is among the more dependable character actors in town (True Colors' "Detroit '67," Actor's Express' "Marcus; or the Secret of Sweet"), but it's an undeniable pleasure to watch him here as a bona fide leading man, and such a romantic one at that. In his most shining moment, he articulates Roland's "unfailing clarity of purpose" with remarkable warmth and depth.

Prior to leaving Atlanta for a five-year teaching job in India, Irby had been a fixture on the local scene, usually cast as musical-comedy ingénues. Recently returned (and last on view in Georgia Ensemble's "Calendar Girls"), she has now matured into a resourceful dramatic actress, investing the otherwise intimidating Marianne with an appropriate "quiet elegance."

In the grand metaphysical scheme of things, “Constellations” often seems to get lost in the cosmos. On a more elemental level, though, in portraying a sweet and possibly never-ending love story, Horizon’s co-stars ring true.

THEATER REVIEW

“Constellations”

Grade: B

Through Feb. 26. 8 p.m. Wednesdays-Fridays; 3 and 8:30 p.m. Saturdays; 5 p.m. Sundays (excluding Feb. 5); 2 p.m. Sunday (Feb. 5 only). $25-$30. Horizon Theatre, 1083 Austin Ave. (in Little Five Points), Atlanta. 404-584-7450, www.horizontheatre.com.

Bottom line: Not quite heavenly, but brightly performed.