On a beautiful and cloudless day, a middle-aged couple celebrate their union with a picnic. Joe Rose and his long-term partner Clarissa Mellon are about to open a bottle of wine when a cry interrupts them. A hot-air balloon, with a 10-year-old boy in the basket and his grandfather being dragged behind it, has been ripped from its moorings. Joe immediately joins several other men in an effort to bring the balloon to safety. In the rescue attempt, one man, John Logan, dies.
Another of the would-be-rescuers is Jed Parry. Joe and Jed exchange a passing glance, a glance that has devastating consequences and that indelibly burns an obsession into Jed's soul, for Jed suffers from de Clerambault's syndrome, a disorder that causes the sufferer to believe that someone else is in love with him or her. Delusional and dangerous, Jed gradually wreaks havoc in Joe's life, testing the limits of his beloved rationalism, threatening Clarissa's love for him, and driving him to the brink of murder and madness.
Joe goes to meet Jean Logan (John Logan's wife) at her Oxford Home. Jean Logan does not want to hear about how her husband was a hero, but Joe tells her that her husband was a brave man acting out a fatherly instinct to protect a vulnerable child. Mrs Logan brings a bag through to Joe which holds a picnic and a scarf smelling of rose water and asks Joe how many doors were open on his car. She accuses her dead husband of having an affair with another woman and asks Joe to phone other people at the accident to ascertain if they had seen anyone with him.
During a lunch with Clarissa and her godfather, Joe witnesses the attempted murder of another man, resulting in the man being shot in the shoulder. However, he realises that the bullet was meant for him and that the similar character of the people at the other table had misled the killers into thinking the other man was their target. Before the hit-man can deliver the fatal shot, Jed intervenes to save the innocent man's life before fleeing from the scene. In the subsequent interrogation, Joe insists that it was Jed who was behind this, but the detective does not believe him, possibly because he appears to get many of the facts of the incident incorrect. Joe leaves dissatisfied, knowing that Jed is still out there and looking for him. Like the detective, however, Clarissa becomes sceptical that Jed is stalking Joe and that Joe is in any danger. This, plus the stress Joe suffers at Jed's hands, strains their relationship.
Fearing for his safety, Joe purchases a gun through an acquaintance. On the journey home, he receives a call from Jed, who is at Joe's home with Clarissa. Upon arriving at his apartment, Joe sees Jed sitting on the sofa with Clarissa. Jed then asks for Joe's forgiveness, before taking out a knife and pointing it at his own neck. To prevent Jed from killing himself, Joe shoots him in the arm.
They then go to meet Mrs Logan and take her and her children on a picnic to which they have invited the woman that her husband was suspected of having an affair with. It turns out that on the previous occasion the young woman had been with her university lecturer with whom she was having a secret relationship; Mr Logan had simply offered them a ride to the picnic when their car broke down. The novel ends with the children and Joe in the river, Joe telling them a story about how the river is made up of many particles.
In the first of the novel's appendices (a medical report on Jed's condition) we learn that Joe and Clarissa are eventually reconciled and that they adopt a child. In the second (a letter from Jed to Joe), we learn that after three years, Jed remains uncured, and is now living in a psychiatric hospital.