Benjamin Braddock, going on from twenty to twenty-one years old, has earned his bachelor's degree from an unnamed college in the Northeast and has returned home to a party celebrating his graduation at his parents' house in Pasadena, California. Benjamin, visibly uncomfortable as his parents deliver accolades and neighborhood friends ask him about his future plans, evades those who try to congratulate him. At her insistence, he drives Mrs. Robinson, the neglected wife of his father's law partner, home. Once at the Robinson home, Benjamin is coerced inside to have a drink and Mrs. Robinson attempts to seduce him. She invites him up to her daughter Elaine's room to see her portrait and then enters the room naked. But her initial attempt at an affair is rebuffed and Benjamin leaves. However, after a few days, he clumsily organizes a tryst at a hotel, thus beginning their sexual relationship.
Benjamin spends the remainder of the summer drifting around in the pool by day, purposefully neglecting to select a graduate school, and seeing Mrs. Robinson at the hotel by night. He discovers that he and Mrs. Robinson have nothing to talk about and that she only wants sex. However, after Benjamin pesters her one evening, Mrs. Robinson reveals that she is in a loveless marriage because she errantly became pregnant with her daughter, Elaine. Both Mr. Robinson, who is unaware of his wife's affair, and Benjamin’s parents encourage him to call on Elaine. Benjamin is forced to date Elaine, but he consciously tries to sabotage his first date with her by ignoring her, driving recklessly, and taking her to a strip club. After Elaine runs out of the strip club in tears, Benjamin has a change of heart, realizes how rude he was to her, and discovers that Elaine is someone he is comfortable with. A relationship ensues.
Trying to stave off a jealous Mrs. Robinson who threatened to reveal their affair to destroy any chance with Elaine, Benjamin rashly decides he has to tell Elaine everything. Upset over hearing about Benjamin's tryst with her mother, Elaine escapes to Berkeley and refuses to speak with him. He follows in pursuit and, after briefly stalking her, reveals his presence. Elaine accuses Benjamin of raping her mother while she was drunk, refusing to believe that it was in fact Mrs. Robinson that craftily seduced him and initiated the affair. After much discussion and over the next few days, Benjamin and Elaine grow closer, and he continually asks to marry her. Mr. Robinson arrives at Berkeley, with all the details of his wife’s affair, where he meets Benjamin in his apartment. He does not know whether he can prosecute Benjamin, but he thinks he can and threatens to have him behind bars if he sees his daughter again. Mr. Robinson then forces Elaine to drop out of school and takes her away to marry Carl, a classmate with whom she had briefly been involved.
Returning to Pasadena in search of Elaine and Mr. Robinson, Benjamin forces himself into the Robinson home but encounters Mrs. Robinson. She coldly tells him he won't be able to stop the wedding and then calls the police, claiming that her house is being burgled. Benjamin returns to Berkeley. After learning from Carl’s fraternity brothers that the wedding is in Santa Barbara, California that very morning, he rushes out to stop the wedding. Running out of gas a few blocks from the church, Benjamin must sprint the remaining distance. He arrives just as the bride and groom are about to kiss. Realizing the ceremony is concluding, he bangs on the glass at the back of the church and screams out "Elaine!" repeatedly. After a brief hesitation, Elaine screams out "Ben!" and starts running towards him. A brawl ensues as guests try to stop Elaine and Ben from leaving together. Elaine manages to break free from her mother, who claims "It's too late!" since Elaine has already said her marriage vows and kissed, to which Elaine replies, "Not for me!"; Mrs. Robinson then slaps Elaine. Benjamin holds guests at bay by swinging a cross ripped from the wall, he then uses it to jam the outside door while the pair escape. They run into the street to flag down a passing bus and take the back seat, elated at their victory. However, in the final shot, Benjamin's smile gradually fades to an enigmatic, neutral expression as he gazes forward down the bus, not looking at Elaine. Elaine first looks lovingly across at Ben but notices his demeanor and turns away with a similar expression as the bus drives away, taking the two lovers towards a future of uncertainty.