Harry Sanborn is a wealthy New York record company owner who only dates women under 30, including his latest girlfriend, Marin Klein. The two drive to her mother's Hamptons beach house expecting to be alone. However, her mother, a playwright named Erica Barry, and her sister Zoe, unexpectedly arrive.
After an awkward dinner, the night turns disastrous when—during foreplay with Marin—Harry has a heart attack and is rushed to a hospital. The attending doctor, Julian Mercer, tells Harry to stay nearby for a few days, so Harry reluctantly stays with Erica. Their personalities clash and create awkward living arrangements at first, but they soon find charming qualities in each other and begin to form a connection. Harry's relationship with her daughter and Erica's budding relationship with Julian become obstacles to their growing attraction. Marin tells her mother that she will break up with Harry, but he ends things first. Harry and Erica spend more time together and eventually have sex. Julian tells Harry he has improved enough to return to the city. He and Erica share an awkward goodbye, as despite their strong feelings, Harry is clearly hesitant to enter into a serious relationship.
Marin receives news that her father and Erica's ex-husband, Dave Klein, is getting remarried to a doctor who is only two years older than her. Though Erica is unaffected by the news, Marin is devastated and pressures her mother into accompanying her to a dinner. At dinner, Erica sees Harry at another table with another, much younger woman. An argument follows and Erica admits that she is in love with Harry, but he does not reciprocate, so she ends things between them and leaves. Harry suffers from what he believes is another heart attack but in the emergency room he is told it was a panic attack.
Devastated, Erica returns home where she cries almost nonstop for several weeks, pouring her heartbreak into a new play about a woman who falls in love with her daughter's boyfriend, titling it "A Woman to Love," a phrase Harry had used to describe Erica. Harry hears about the play and rushes to the theater where it is being rehearsed. Despite her denials, it is obvious that she has used the most personal details of their affair in the play. When he tells her he still cares about her, Erica rebuffs him. After learning his character dies in the play, he suffers another panic attack. At the hospital, he is told he needs to de-stress and he relocates temporarily to the Bahamas.
Six months later, Erica's play is a huge success. Harry pays Marin a visit to apologize for any past disrespect and discovers he never did and that she is now happily married and pregnant. Marin informs Harry that Erica is in Paris celebrating her birthday. Harry flies to Paris and surprises Erica at her favorite restaurant. He tells Erica that he has been reaching out to all the young women he had affairs with in an attempt to atone for his heartless behavior. Julian, whom Erica is now dating, appears.
Harry, Erica and Julian have dinner together, and part amicably outside the restaurant. While Harry gazes in heartache over the river Seine, Erica arrives. She tells him that Julian realized she still loves Harry and decided to step aside to let them be together. Harry tells her that at age 63, he's in love for the first time in his life, and they embrace.
A year later at a restaurant, Erica and Harry, now married, are out with Marin, her husband Danny and their new baby daughter, celebrating life as a loving family.
Jack Nicholson as Harry Sanborn
Diane Keaton as Erica Barry
Keanu Reeves as Julian Mercer
Amanda Peet as Marin Klein
Frances McDormand as Zoe
Jon Favreau as Leo
Paul Michael Glaser as Dave Klein
Rachel Ticotin as Dr. Martinez
KaDee Strickland as Kristen
Peter Spears as Danny Benjamin