Millicent is a middle-aged spinster in Los Angeles with no apparent friends other than her landlady, from whom she rents a bungalow apartment and works as an at-home typist to pay the bills. She does have a great love of music, and one night after a concert an imposing young man named Burt introduces himself. While at first she is understandably cautious, he quickly charms her with tales of his humble upbringing and military service in Japan. Millicent-- whom he quickly calls Millie-- warms to him, and he is almost instantly in love with her.They soon agree to go swimming at the beach, where Millie reveals her rather impressive figure in a bathing suit. They frolic in the sea, and Burt brings her back onto the shore, embracing her on the sand and kissing her as the waves roll in around their bodies (in a virtual reverse shot of the more famous beach love scene in 'From Here to Eternity' a few years earlier).Burt asks her to a movie, and in quick order tells her he wants to get married. She is understandably distraught, pointing out their age difference is rather extreme (never specifically stated, but at least 20 years), and encouraging him to consider younger women. He is not swayed, maintaining his love, but she sends him away, insisting they should never see each other again.A month goes by in which Millie does her typing and spends time at lonely chores, until she becomes elated when Burt returns for a visit. His love remains strong, and so does hers; after one more attempt to resist him, she agrees to drive down to Mexico with him and get married overnight.They honeymoon for a week, and soon Millie becomes concerned when some details that Burt mentions about his past begin changing. Those concerns are minor compared to the arrival of his first wife, Virginia, whom Millie did not know existed. Since Burt is not there, Virginia asks Millie to have him sign the final documents from their divorce making Millie realize she is a party to bigamy-- and alludes to his troubles with compulsive lying. She also mentions Burt's father, who is not dead as Burt claimed but visiting L.A. at the time.Millie goes to visit Burt's smarmy father, who further alludes to his son's frustrating mental troubles, but can not explain them. Millie does not know, however, that Virginia is also staying in his hotel room, and the two of them are clearly carrying on a long-term sexual relationship.Millie confronts Burt about his lies, and as she moves on to discuss his father, he grows more irate. She nonetheless convinces him to go visit his father at the hotel, and secretly follows him. There, Millie sees that Burt's father and Virginia are in fact canoodling, but she is too late to stop Burt from seeing they are sharing the same room.Millie brings Burt back to their apartment, and he is understandably devastated. Burt's dad and Virginia soon arrive and Millie confronts them with the truth of their terrible deception and disregard for Burt. Millie realizes that they are trying to coerce Burt into the divorce so that he will sign away his deceased mother's valuable property to the two of them.After sending away the scoundrels without their desired legal documents, Millie tries to explain the situation to Burt. Alas, Burt escalates to such a level of anger and paranoia-- believing that Millie is in cahoots with his dad and ex-wife in the scam-- that he slaps Millie and ultimately tries to slam her typewriter on her, which lands on her hand.While Millie seeks treatment for her injuries from the beating, a doctor recommends that she should consult a psychiatrist about Burt. She goes to visit the pompous shrink, who insists that Burt needs months of institutionalization, regardless of whatever the hell is wrong with him. Millie remains faithful that somehow she will not need to make that choice.Within a short time though, Burt has such terrible episodes that Millie signs for him to be committed, and two guys in white jackets come to forcibly take him away to an asylum. While he is there, the authorities subject him to multiple shock treatments and force abundant drugs into him, and Millie waits for months with a guilty conscience, worrying that Burt will not be cured, and accepting that if he is, he will no longer want to be married to her.After the many months go by, Millie is ordered to pick up Burt at the sanitarium, but she only wants to appear briefly to release him and wish him well as he moves on. When they meet, Millie blathers on again for some time about how she is too old for him, and now that he is well he can get on in life with a younger woman. As she walks him off the grounds, he asks her to show him the hand that he smashed with the typewriter-- an incident he had repressed in his previous mania. She lifts her hand and he kisses it, then they deeply kiss each other. Someone walking by gives them a long critical look, which they notice, but they agree they do not care what other people think.They walk off into the falling autumn leaves (with Nat King Cole singing along).