COOS BAY — They’re almost the prototypical couple of the 1950s: A businessman and a housewife, parents to a bunch of kids.

But George and Doris are different in one crucial way: They’re both married to other people.

The nostalgic image of the era doesn’t usually include adultery, but it’s central to the romance of “Same Time Next Year,” an On Broadway Thespians production opening tonight at Gussie’s, in the dining room behind the Coos Bay bar.

The relationship started with a one-night stand. In the first minutes of the play, George (Jeff Roberts) calls Doris (Leatha Lewison-Gonzalez) “Dorothy” because he can’t remember her name. But already he feels he’s falling in love.

The only problem is that his marriage is a happy one. He also loves Helen, his wife, and Doris can hardly think of a flaw to pin on her husband, Harry. But nor can they deny their feelings for each other. There’s just one option left: continuing the affair. Choices are so easy in romantic comedies.

But it isn’t easy for them to get together when they live on opposite coasts, George in New Jersey and Doris in Oakland, Calif. As they sort through their morning-after feelings in a hotel room at the Sea Shadows Inn in Northern California, they find it wasn’t fate that brought them there the same weekend.

For George, it’s the annual ritual of a business trip to meet with his first client in the wine country. For Doris, it’s a Catholic retreat in the area that allows her to get out of going with her husband and kids to visit his mother in Bakersfield on her birthday.

With their schedules so conveniently aligned, they might have met sooner, but it’s 1951 when George and Doris meet and discover their built-in excuse to return for another hookup the following year. It becomes an annual tradition: “One beautiful weekend every year with no cares, worries or responsibilities,” as George puts it.

As long as they confine their visits to that one weekend a year, they won’t have to make up any new lies to tell their spouses. Once a year isn’t often for lovers to see each other, but it worked for the cowboys in “Brokeback Mountain,” a 2005 film in which fishing trips sustained a 20-year romance.

But it isn’t the next year when George and Doris are next seen; it’s 1956. The play moves forward in five-year increments, taking a longer view of a lifelong relationship without extending the running time beyond two hours, first-time director Cherish Merritt said.

At each reunion, the lovers share news of their lives and their families — evidently, they don’t communicate at all between visits, so there’s always something to catch up on. These details solidify the friendship at the core of the relationship and foreground the difficulty of balancing their family lives and the affair.

The emphasis on the emotional connection and the characters’ unwavering commitment to their family responsibilities goes a long way toward cleaning up a play about adultery, but the obligatory moments of making out, heavy breathing and partial nudity bring a warning for adult content.

As the years progress, both George and Doris take unexpected turns as they adjust to the changing times of the ’60s and ’70s, often moving in opposite directions, but always returning to each other.

The play continues through June 29. Performances are at 6 p.m. Fridays and Saturdays and at 2 p.m. Sundays. Meals are available but not required. Tickets are $10, $7 for children and $8 for matinees.