Archibald Bannister, a "germ detective" for WHO (World Health Organization), detains oilman Steven Cooper for observation at the London airport to diagnose a disease contracted by Cooper. Accompanying Cooper is his bride of two weeks, Michèle. Also at the airport is Edward Kennedy, Cooper's American partner, who is puzzled by the negative report Cooper submitted on oil-drilling tests carried out in the Middle East. Later, at a party given by unscrupulous oil tycoon Nick Ivanovitch, Kennedy is snubbed by his host after requesting more funds for additional drilling. The party is interrupted by Bannister, who announces that Cooper has smallpox and that all the guests must be vaccinated. Back at WHO headquarters, Bannister finds reports of two other smallpox cases. After questioning Michèle about Cooper's activities, he becomes convinced that all three cases are somehow related. He and Kennedy join forces to solve the mystery which leads them to an Alpine village. Here they discover that a missing document belonging to Cooper is the carrier of smallpox. With this new information, Kennedy unmasks Ivanovitch as the brains behind an oil swindle. Following Cooper's death, Kennedy and Michèle decide to form a professional--and romantic--alliance.