As Stephen Hawking is fond of reminding us, there’s nothing in the laws of physics that prohibits time travel. Playwright Jason Robert Brown has applied the dramaturgical equivalent of that principle to his musical “The Last Five Years,” the central conceit of which is that the arrow of time points in opposite directions for the play’s characters, two young lovers.

Camelot Theatre opened a spare, energetic production of the two-person play Friday night, directed by Livia Genise, with Nathan Monks as hotshot young Jewish novelist Jamie Wellerstein and Amanda Andersen as his shiksa goddess, Catherine Hiatt.

The play is the story of the five-year romance of Jamie and Catherine, whose arcs move opposite one another in time. Jamie is thrilled at meeting Catherine, falls in lust, has qualms, disappoints his mother (who would prefer a nice Jewish girl), watches his career take off, falls in love, has second thoughts, moves on to infidelity and lies.

Meanwhile, we watch Catherine experience the relationship a la “The Backwards Episode” of “Seinfeld” in which Jerry, George and Elaine travel to India for Sue Ellen Mischke’s wedding and the narrative unfolds from the present to the past.

Catherine begins, in our time, with a broken heart (“Still Hurting”), becomes worried but hopeful, falls in love, struggles with her insecurities, shows youthful naiveté and gets excited over meeting Jamie.

The double-timelines gimmick can be a bit befuddling, but the plot never quite falls apart. In fact, forcing us to try to keep the timelines straight is what gives the play its peculiar charm. The characters are sharply drawn and brought to convincing life by the actors, both of whom are fine singers. In our time, which is obviously Jamie’s time, since the alternative isn’t available to us, it becomes clear that he’s self-absorbed, perhaps even narcissistic.

In Catherine’s reverse time it’s equally clear that she’s needy, maybe a little neurotic. The only time the characters meet is in the middle, when they get married and have their only duet (“The Next 10 Minutes”).

Brown, who wrote the music and lyrics as well as the book, obviously worships, as he confesses, at “the church of Steve” (Sondheim). The songs are pop/soft rock tinged with classical influences. The lyrics are conversational and filled with angular harmonies and verbal interplay. They are never merely pleasant tunes and always serve the story. The score is handled with elan by keyboardist/orchestrator John Taylor, violinist Beth Martin and cellist Sue Flynn, who sit and play upstage throughout.

In “Shiksa Goddess,” Jame knows he’s breaking his mother’s heart and can’t help it. In “See I’m Smiling,” Catherine hopes against hope that the relationship can be salvaged. Catherine’s best song is perhaps “Climbing Uphill,” in which she reveals her insecurities.

When a marriage goes on the rocks, whose fault is it? Catherine is clearly losing it as Jamie's book gets a positive review by John Updike while she can’t get her career on track, as we see in a comic audition. But it’s Jamie who has an affair, or at least a fling, and seems to pull away.

Camelot is known for big musicals, and this time out Genise has done a switcheroo, presenting an intimate little story with lots of verve. Whatever your final take on the dual timeline, or where to place the blame, you’ll have fun getting there.

Reach Medford freelance writer Bill Varble at varble.bill@gmail.com.