Screen adaptation of the successful wartime play, with an all-star cast. A Fifth Column organisation plans to kidnap a scientist (Leslie Banks) from his country house laboratory, and while he is unaware of any threat to him he continues working on his revolutionary new bomb-sight. His assistant (Michael Wilding) is suspected of passing secret information to enemy agents, and the arrival of an injured officer (John Mills), a writer (Alastair Sim) and an evacuee from London (a youthful George Cole) only add to the twists and turns of the plot.