After earning his bachelor's degree from an East Coast college, Benjamin Braddock returns to his parents' Pasadena, California home. During his graduation party, he is urged to join a business dealing in plastics - the material of the future. Benjamin cringes as his parents deliver accolades during the party and retreats to his bedroom until 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 after his parents continue to make him feel isolated and desperate for any kind of connection, he invites Mrs. Robinson to the Taft Hotel, where he registers under the pseudonym "Mr. Gladstone".

Benjamin spends the summer relaxing in his parents' pool by day and meeting Mrs. Robinson at the hotel by night. During one of their trysts, Mrs. Robinson reveals that her loveless marriage resulted when she accidentally became pregnant with her daughter, Elaine. When Benjamin jokingly suggests that he date Elaine, Mrs. Robinson angrily forbids it. However, Benjamin's parents, unaware of the affair, are eager for their son to date Elaine and relentlessly pester him to ask her out, as does Mr. Robinson. Benjamin gives in and reluctantly takes Elaine on a date.

When he sees how upset Mrs. Robinson is, Benjamin attempts to sabotage his date by ignoring Elaine, driving recklessly, and taking her to a strip club. She flees the club in tears, but Benjamin, feeling remorseful, runs out 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 with a married woman. Benjamin swears that the affair is over and makes plans for another date with Elaine for the following day.

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 refuses to see Benjamin again and returns to school at Berkeley.

Benjamin follows her to Berkeley hoping to regain her affections. Elaine is aghast because her mother told her that Benjamin raped her when she was drunk. After Benjamin explains what really happened and apologizes, Elaine forgives him and they rekindle their relationship. He asks her to marry him, but she is uncertain despite her feelings for him. Later, an angry Mr. Robinson arrives at Berkeley and confronts Benjamin in his boarding 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 Smith, a classmate whom she briefly dated.