After earning his bachelor's degree from an East Coast college, Benjamin Braddock returns to his parents' Pasadena, California, home. During his graduation party, hosted by his parents, Benjamin cringes as they and their guests praise him, and he retreats to his bedroom, where Mrs. Robinson, the wife of his father's law partner, insists that he drive her home. Once there, she tries to seduce him. He initially resists her advances, but later changes and soon invites Mrs. Robinson to the Taft Hotel, where he registers under the surname, "Gladstone".
Benjamin spends the summer floating in his parents' swimming pool by day and meeting Mrs. Robinson at the hotel by night. During one of their trysts, Mrs. Robinson reveals that she and her husband married after she accidentally became pregnant with their daughter, Elaine. When Benjamin jokingly suggests that he date Elaine, Mrs. Robinson angrily forbids it. However, Benjamin's parents and Mr. Robinson, unaware of the affair, are eager for Benjamin to date Elaine, and relentlessly pester him to ask her out. Benjamin gives in, and he reluctantly takes Elaine on a date to Mrs. Robinson's displeasure. Ben attempts to sabotage the date by ignoring Elaine, driving recklessly and taking her to a strip club. She flees the club in tears, but Benjamin, feeling remorseful, goes after her, apologizes, and kisses her. They eat at a drive-in restaurant, where they bond over their shared uncertainty about their future plans. After they visit the Taft Hotel for a late-night drink and the staff greet Benjamin as "Mr. Gladstone", Elaine deduces that Benjamin is having an affair. Benjamin admits his affair was with a married woman he does not name, tells Elaine the affair is over, and asks to see her again.
To prevent Benjamin from dating Elaine, Mrs. Robinson threatens to tell Elaine about their affair. To thwart this, Benjamin reveals to Elaine that the married woman is her mother. Elaine is so upset that she throws Benjamin out of the house. Soon, she returns to school at Berkeley. Benjamin follows her to Berkeley, hoping to regain her affections. Elaine initially rejects him and briefly dates medical student Carl Smith, but then learns that her mother lied to her when she claimed Benjamin raped her, and the pair reconcile. Benjamin pushes for an early marriage, but Elaine is uncertain despite her feelings for him. Later, an angry Mr. Robinson arrives at Berkeley and confronts Benjamin in his boardinghouse room, where he informs him that he and his wife will be divorcing soon and threatens to have Benjamin jailed if he continues to see Elaine. He then forces Elaine to leave college to marry Carl. Benjamin drives back to Pasadena, breaks into the Robinson home searching for Elaine; but, confronts Mrs. Robinson, who calls the police claiming her house is being burglarized. She tells Benjamin that he cannot prevent Elaine's marriage to Carl. Benjamin flees and drives back to Berkeley. There, he discovers the wedding is in Santa Barbara that day. He speeds to Santa Barbara but his car runs out of gas a short distance from the church. Benjamin runs to the church, arriving just as the ceremony is ending. His desperate appearance in the glass church gallery stirs Elaine into defying her mother and fleeing the sanctuary. Benjamin fights off Mr. Robinson and repels the wedding guests by swinging a large cross, which he uses to barricade the church doors, trapping everyone inside. Benjamin and Elaine, in her wedding gown, escape aboard a bus and sit among the startled and staring passengers. As the bus drives on, their ecstatic smiles slowly fade away as they begin to ponder their uncertain future.
