Frank Raftis (Robert De Niro) is an architectural engineer who lives in Westchester, New York, with his wife Ann (Jane Kaczmarek) and their two young sons. Molly Gilmore (Meryl Streep) is a graphic artist who resides in the same suburban county with her husband Brian (David Clennon), a physician; they have no children.

Frank and Molly ride a Manhattan bound commuter train together the day before Christmas. After shopping, they collide at the front door of Rizzoli's Bookstore, and their packages scatter on the floor. After apologies and some small talk, they go their separate ways. On Christmas morning, Ann opens a gift from Frank, and it is a book on sailing; Brian unwraps his present from Molly, and it is a book on gardening. In their separate households, the two strangers share a common chuckle, realizing what has happened.

Three months after Christmas, Frank and Molly bump into each other on the train again. He asks her if she works in the city, and she nervously responds, "I'm married." That evening, Frank finds Molly on the return train and suggests they take the same train in the future. The next day, after visiting her dying father (George Martin) at the hospital, Molly goes out to lunch with Frank. Although nervous with each other, they talk about their lives. But neither of them knows quite what to do with their feelings of attraction for the other.

As Frank and Molly continue to meet in Manhattan, they reluctantly tell friends about their new relationship. Frank's buddy Ed (Harvey Keitel) is divorcing his wife and savoring the freedom of being single again. He chides Frank for feeling guilty about socializing with another woman. Isabelle (Dianne Wiest), who enjoys dating all kinds of men, wishes Molly would have more fun.

As Molly frets over what dress to wear and Frank queries Ed about his appearance, it is evident that these two are thinking constantly about each other and deriving considerable pleasure from being together. However, when they meet in an apartment to make love, Molly cannot go through with it. Then at home in Westchester, she learns that her father died while she was with Frank. Guilt consumes Molly, causing her to have an emotional breakdown, followed by depression. Meanwhile, Ann has noticed Frank's moodiness. With great difficulty, he tells her he's been seeing Molly. "I'm not having an affair," he says. "No, it's worse," Ann replies in anger and hurt.

Falling in Love, directed by Ulu Grosbard, is a delicately crafted and richly acted drama about what psychologist Dorothy Tennov has called "limerance" — a form of romantic love characterized by infatuation, an aching of the heart, and a general intensity of feeling that overshadows other concerns or interests. Limerance includes sex but is manifested mainly as a mental activity. Screenplay writer Michael Cristofer (Shadow Box) accurately portrays this phenomenon as two decently married individuals find their lives upended when they are swept away by romantic love for each other. Cristofer has said of the film: "We all have notions that it is wonderful to fall in love. But it can be disruptive and destructive."