At the height of the Second World War, a group of men and women are gathered together in London to be trained as spies. However, there is a traitor in their midst. A Catholic priest (Robert Beatty) and a Belgian expatriate (Simone Signoret) are among a select group of people chosen by the British for secret operations in Nazi-occupied Belgium during World War II. After parachuting into enemy territory, the agents set out to complete a two-part assignment: destroy German information located in Brussels and rescue a fellow operative from the Nazis. However, when word arrives that one of their group is a traitor, the mission falls into jeopardy.