During the Cold War, an American scientist arrives in London on a secret errand for the CIA. His task is to make contact with a defecting Hungarian scientist who wishes to convey information to the West concerning a powerful virus developed in the East that could be turned into a biological weapon of mass destruction. The Hungarian insists on passing the information solely to an American virologist. The American, Frank Smith, arrives in London escorted by a female CIA agent, Eve Trent. They pose as a newly married couple on honeymoon. During the covert meeting between the Hungarian scientist and the 'honeymooners', in a London subway station, information about the virus and its antidote is passed to the American scientist who memorizes it. Nevertheless, the meeting is monitored by enemy agents. Later, at their hotel, the American 'honeymoon' couple is followed by Russian-speaking enemy agents. Across the street, in a residential building facing the hotel, more agents espy the American couple's hotel windows. These agents are shown speaking in French. During the night, a shot, coming from the Americans' room, is heard by various witnesses and hotel patrons and staff. The American scientist is seen leaving his room in a hurry. His 'wife', Eve Trent, is found shot dead inside the room. Her 'husband', Frank Smith, is arrested by the British police while attempting to catch a flight to Ireland from the airport. Accused of murdering his 'wife', Frank Smith denies any wrongdoing and claims that an armed intruder killed his 'wife' but he admits to have fled the scene in panic. Famous British barrister, Mary Randall, Q.C., is tasked by the court to defend Frank Smith. Unfortunately, her task is difficult because Frank Smith refuses to tell her the truth about his real identity and mission in the UK. Afraid of tipping-off the enemy agents, the CIA officers handling Frank Smith refuse to acknowledge him or assist him in any way. Frank Smith faces the death penalty, if convicted of murder. His only hope is British barrister, Mary Randall, Q.C.