Late one night, Stephen, an Oxford philosophy don, hears a car accident outside his house. He discovers the occupants of the car are two of his students, aristocrat William and William's fiancée, Anna. William is dead, but Anna is well enough to be led back to the house. Stephen phones the police but finds that Anna has slipped upstairs before the police arrive. He tells the police about the accident but does not mention that Anna was in the car. After they have left, he goes upstairs, and, while watching Anna asleep, recalls the events that led up to the accident.
At one of their tutorials, William has mentioned to Stephen his attraction for another of his students, Anna. Guiltily (for he is married and his wife, Rosalind, is about to give birth to their third child), Stephen also becomes infatuated with her. Walking across the campus one day, he is hailed by William, who is with Anna and who invites him to join them in a punt down the river. The occasion ends ignominiously for Stephen when he overbalances and falls in the water - a reminder, he feels, of his lack of co-ordination and advancing years. He invites William and Anna over to his house for Sunday lunch.
On the Sunday, Stephen is surprised to find that his invited guests have been joined by Charley Hall, a fellow don who is also a successful novelist and media personality. As the day develops, tensions begin to surface. A playful tennis match brims over with sexual rivalry. Charley mischievously implies that Stephen is having an affair with a student at Oxford. Anna initiates a walk with Stephen, but, when alone with her, he is afraid to make a move. The evening ends in disarray, with Stephen taunting Charley with news of his forthcoming appointment with a television producer and the guests having to stay overnight because of their drunken condition.
Stephen arrives at the BBC only to find that the producer he was due to see has been rushed to hospital. A BBC employee who was at Oxford with Stephen asks him if he still sees Francesca, the Provost's daughter, who Stephen, it seems, was courting before he met Rosalind. On an impulse, he phones Francesca, who now lives alone. They go out for dinner and then spend the night together.
He returns home. Expecting the house to be empty, with Rosalind and the children at Rosalind's parents prior to her confinement, he is startled to discover Charley and Anna emerging from his bedroom. They have broken into the house, and reveal that they are lovers. Stephen gives Charley a key to the house. However, when he tells Rosalind about the affair, she is furious. That weekend he is a guest at William's home, where he is compelled to participate in a brutal indoor game of mock rugby (the Eton wall game) which ends with him and William locked in a battle of strength. Later, at a cricket match at Oxford, he is approached by Anna, who tells him that she and William are to be engaged: would he tell Charley for her? The young couple run off, with William having arranged to drive round with Anna later that night to see Stephen.
Back in the present, Stephen's frustration breaks and he forces himself on Anna, who is still in shock and who Stephen suspects was actually driving the car. The next morning he has an urgent phone call from the hospital asking him to visit his wife. Dropping Anna off at her college, he hurries to the hospital: Rosalind tells him that their new-born child is having difficulty breathing. Later that day, he encounters Charley, who has just heard about the accident and that it was Stephen who had discovered William's body (he does not know Anna was in the car). They visit Anna in her college room, to discover that she is packing to return home. She bids a casual farewell to Charley. Stephen returns home. As he and the children play in the ground outside their house, the sound of an accident is heard.
