Joe Rose and his wife, Clarissa Mellon, are picnicking in the English countryside when they hear the shouts of a child in distress. The child, Harry Gadd, is in the basket of a hot-air balloon, which the wind is threatening to carry away. His grandfather, James Gadd, is working feverishly to secure the basket to the ground. As Joe is racing toward the balloon in an attempt to help, he is joined by several other men, among them John Logan, a local doctor and former mountain-rescue worker, and Jed Parry, a young man who lives alone on the income from a large inheritance. Though the men do their best to provide assistance, taking hold of the ropes dangling from the balloon’s basket, their intervention ends in disaster. John Logan holds on to his rope when a burst of wind carries the balloon high into the air and, to the horror of everyone present, he falls a great distance to his death.
In the moments after Logan’s fall, Joe and Parry share a few minutes together as they wait for the police to arrive. Parry encourages Joe to pray, and when Joe responds that he holds no religious beliefs, Parry is increasingly insistent. Later that evening, after Joe and Clarissa have talked through the events of the day again and again, Joe is awakened by a phone call. On the other end of the line is Parry, who insists that he understands what he believes Joe to be feeling and that he loves Joe, too. Confused and flustered, Joe hangs up the phone and tells Clarissa that the call was a wrong number.
In the days that follow, Parry’s behavior grows increasingly perplexing. He suffers from de Clerambault’s syndrome, which has given him the delusion that he and Joe are in love, and, as a consequence, he begins to write Joe long letters, follow him in the streets around Joe’s apartment, and leave pleading telephone messages on Joe’s answering machine. Though Joe attempts to explain to Clarissa what is happening, she is hesitant to believe that Joe is in any danger, preferring instead to think that Parry is harmless and ought to be gently and carefully reasoned with.
In part to escape from Parry for a few hours, Joe travels to Oxford to visit John Logan’s widow, Jean Logan. Distraught and inconsolable, Jean reveals her belief that her husband was having an affair in the weeks before his death. She questions Joe about the afternoon of the accident and threatens to kill her husband’s supposed lover if she ever meets her. Back in London, Joe finds his relationship with Clarissa to be increasingly troubled. Parry’s obsession has caused a rift between the two of them, and an atmosphere of mutual distrust has arisen in their household.
Things continue in this manner until the afternoon of a birthday luncheon in Clarissa’s honor. Joining her and Joe is Clarissa’s godfather, an elderly scientist and professor. As their meal progresses, Joe notices a similarly composed group—a woman and two men—dining at a nearby table. Suddenly, a pair of gunmen enter the restaurant, move toward the nearby table, and shoot the younger of the two men sitting there. Before they can shoot him a second time, however, a man whom Joe recognizes as Jed Parry intervenes. Parry has sent the men into the restaurant to kill Joe, but they have mistakenly shot a man Joe’s age.
Unsatisfied with the response of the police, who cannot be convinced that Joe is in danger despite what has happened, Joe purchases a gun from a former friend. On his way home, he receives a call from Jed Parry, who tells Joe that he is sitting in Joe’s apartment with Clarissa and that Joe must join them right away. Racing back to London, Joe finds that Parry and Clarissa are indeed together. A distraught Parry confesses that his love for Joe has ruined his life, and when he pulls a knife from his pocket, Joe shoots him in the arm to prevent him from killing himself.
In the novel’s closing pages, Joe and Clarissa travel to Oxford once more to visit Jean Logan. They picnic with Jean and her children beside a river and are joined by two of John Logan’s friends: a university lecturer and the young woman with whom he is romantically involved. John Logan’s supposed affair, the university lecturer reveals, did not occur. Rather, John was giving the lecturer and the young woman a ride in his car when he stopped to assist the balloonists, a fact that led to the circumstances and details that aroused Jean’s suspicions.
Simultaneously relieved and guilty, Jean Logan wonders who can forgive her for doubting her husband’s faithfulness. Her question makes Joe and Clarissa ponder their own relationship, and while Joe concedes that he might one day forgive Clarissa for discounting the threat posed by Parry, he isn’t yet able to do so.