When the existence of a strain of plague (disease) (vaguely identified as pneumonic plague) is revealed at the US mission at the International Health Organization, three terrorists seek to blow up the US mission. Two of them are shot, one mortally, by security personnel but one escapes. The surviving terrorist is hospitalised and quarantined and identified as Sweden. Dr. Elena Stradner (Ingrid Thulin) and US Colonel Stephen Mackenzie (Burt Lancaster) (Military Intelligence assigned to the IHO) argue over the nature of the strain, which Stradner suspects is a biological weapon but which Colonel Mackenzie claims was in the process of being destroyed.