Leonard Kraditor is walking along a bridge over a stream in Brooklyn, when suddenly he jumps into the water in an attempted suicide. He changes his mind and quickly walks to the home where he resides with his parents. His mother Ruth, seeing him dripping wet, tells her husband Reuben their son has tried it again and it becomes evident that Leonard has attempted suicide before.
His parents tell him that a potential business partner and his family are invited for dinner that night and ask him to be present. When they arrive, Leonard finds that he had been set up with the other family’s daughter, Sandra Cohen. She inquires about his interest in photography and notices a photo of a girl above his headboard. He explains he had been engaged to the girl for several years, but the relationship was broken off when it turned out both he and his fiancée carried the gene for Tay–Sachs disease, which results in diseased children who generally don’t live beyond age of five years, so they would be unable to have healthy children.
Leonard meets a new neighbor, Michelle Rausch, and is immediately attracted to her, choosing to ignore that she is a drug addict. He learns that she is dating a married partner in the law firm where he, Ronald Blatt, practices. At her request, Leonard agrees to meet the pair for dinner at a restaurant. The couple leave him later that evening, as they have plans to attend the Metropolitan Opera. Leonard returns home dejected, but surprisingly, Sandra arrives, sent over by Leonard’s parents. Believing that Leonard wanted her to come by, she realizes by his shocked expression that she was set up. She apologizes for the misunderstanding and says that, if he isn’t interested, a lot of other guys are. Leonard says that he likes her, and they kiss and eventually make love, and with time, his relationship with Sandra deepens.
Michelle calls Leonard and says she is sick. He takes her to the hospital, where she has a D&C for a miscarriage. She had been unaware she was pregnant and is even more angry that Ronald ignored her calls. Leonard takes her home but hides when Ronald arrives unexpectedly. Ronald apologizes to Michelle for not having come to her aid, but Michelle coldly asks Ronald to leave and then asks Leonard to write something on her forearm with his finger while she falls asleep. Leonard writes “I love you”.
Two weeks later, Michelle meets Leonard on the roof of their building and tells him that she has broken up with Ronald and is going to San Francisco. Leonard tells her not to leave and professes his love for her. They make love and plan to leave together the next day for San Francisco.
On New Year’s Eve, Leonard buys an engagement ring for Michelle. Sandra’s father Michael then summons him and offers him a partnership in the family businesses, assuming that he is going to marry Sandra. Noticing the jeweler’s gift bag Leonard is holding, the father assumes it is for Sandra; Leonard lies that it is.
During his parents’ New Year’s Eve party, Leonard hides in the courtyard to meet Michelle. Michelle arrives late and tells Leonard that she isn’t going to San Francisco, because Ronald, having learned Michelle is leaving him for California, has decided to leave his wife and children for her. Disheartened, Leonard permanently breaks things off with her.
Feeling depressed, Leonard heads to the beach, intending to kill himself. When he drops a glove that Sandra had bought for him, he realizes that, in Sandra, he has found someone who loves him and with whom he can build a happy life. He picks up the glove and sees the boxed engagement ring lying on the sand, where he had thrown it from the boardwalk moments before. He returns to the party, where he gives Sandra the ring and embraces her in a tearful passionate hug.
