John, the narrator, a former astronaut, has been hired to investigate the death of a fellow American named Adams, who has died of unknown causes in Naples. This is only the most recent in a series of twelve strange deaths in which the victim first exhibited evidence of great excitement and aggressive behavior, followed by hallucinations and delusions of persecution, and, finally, total withdrawal, leading to death, in most cases by suicide. Though the twelve were unknown to one another, John believes that the pattern cannot be mere coincidence. Were they the victims of a great, mysterious conspiracy?

In an attempt to discover precisely what caused the death of Adams, John, monitored by two colleagues who follow at a distance, duplicates exactly the movements of Adams in Naples and Rome, hoping to tempt the presumed killer to attack him. He stays at the same hotel, drives the same highway from Naples to Rome, stops at the same service station, and registers at the same hotel in Rome. Although he is suspicious of some of the things that happen to him—a young woman in the service station, for example, approaches him and then faints—he learns nothing that can explain what happened to Adams.

He decides, therefore, to go to Paris to consult with Dr. Philippe Barth, a distinguished computer scientist who has been programming a computer to solve problems in which the amount of data exceeds the storage capacity of human memory. At the Rome airport, however, John is delayed when he saves a young girl from a terrorist’s bomb which kills several people. At first he is arrested as the terrorist’s accomplice; then, as a hero, he must endure a news conference, though he wishes to be anonymous. Eventually, he is able to deliver the girl, Annabella, to her father in Paris.

In Paris, he meets Barth, to whom he describes the twelve deaths and the problem inherent in the fact that they seem simultaneously related and unrelated. Barth is convinced that whoever is responsible for the deaths in Naples wants to create the impression that he does not exist; Barth believes that the only way to discover the motive and the perpetrator’s method is to examine all the elements of the pattern. All the victims, for example, were bald, or balding, and all of them suffered from allergies, for which they were taking an antihistamine which contained the stimulant Ritalin.

When John leaves Barth’s institute, his car is sideswiped, and he realizes that if he had been killed his death would have fit the Naples pattern. Later, at a party given by Barth, he meets a police inspector who tells him about the case of an optician who attempted to throw himself into the Seine and later died of heart failure. It was discovered that the optician had repaired the eyeglasses of a chemist who was doing research for the French government into depressants for chemical warfare. The glasses were contaminated by the drug. This story leads Barth and John to consider the possibility that the Naples victims were objects of an experiment by some secret agency that was testing some drug as part of a plot, perhaps to assassinate public officials in Italy.

To test this hypothesis, John prepares to return to Italy. Without realizing it, however, he commits a series of acts which finally explain the mystery. That night he eats some almonds, then sleeps in a bed in which Barth’s superstitious mother, who has no confidence in his antihistamine, has sprinkled “flowers of sulfur” to cure his hay fever. The next day, at the airport, he...