Secret Agent Ted Spencer goes undercover as a grocer to expose a band of spies operating out of a London movie theater.  The theater owner, Verloc, asks Stevie, the young brother of his innocent wife, to deliver a package of films, which actually contains a bomb.

When Stevie is delayed, he is killed when the bomb explodes in a crowded bus.  Mrs. Verloc avenges her brother's death by killing her husband, but her guilt is hidden when the movie theater itself is blown up by another saboteur.