They couldn't have celebrated happier anniversaries if they were married to each other.

In 1951, at an inn located on the Mendocino County coast, Doris (Ellen Burstyn), a 24-year-old housewife from Oakland, meets George (Alan Alda) a 27-year-old accountant from New Jersey at dinner. They have a sexual tryst, and then agree to meet once a year to rekindle the sparks they experience at their first meeting, despite the fact that both are happily married, with six children between them. They each discuss their respective yet unseen spouses, "Harry" and "Helen."
Over the course of the next 26 years, they develop an emotional intimacy deeper than what one would expect to find between two people meeting for a clandestine relationship just once a year. During the time they spend with each other, they discuss births, deaths, including George's son Michael dying in Vietnam, which changes George politically, and marital problems each experiences at home, while they adapt themselves to the social changes affecting their lives.
At their meeting in 1977, George explains that his wife, Helen, has died of cancer earlier in the year, and revealed to a friend that she had known of the affair for 10 years without telling George. Now a widower, George proposes to Doris who refuses to accept because of her loyalty to, and respect for, Harry. Rejected, George leaves for good, only to return with the promise to continue the affair as long as they are able.