A ring of saboteurs is causing havoc in London with a series of explosive terrorist attacks. Karl Verloc (Oscar Homolka) is part of the group, but he maintains a cover as a kind movie theater owner. His wife (Sylvia Sidney) is beginning to suspect something, though, and so is Scotland Yard Detective Sgt. Ted Spencer (John Loder). What neither of them know, however, is that Verloc uses his wife's little brother (Desmond Tester) to deliver the bombs in film canisters.