Ted (John Loder) is loyal and tough Scotland Yard police officer who is tasked with going undercover and infiltrating an intelligent and careful group of foreign spies. The man in charge of this group is Verloc (Oskar Homolka), who along with other members of his small family run a tiny theater. While his family is unaware of his secret job, Verloc spends his days formulating plots to cause terror and panic amongst the British people. He is then tasked with detonating a bomb on the subway, a job which he gives to his brother. Now Ted must find the bomb and stop Verloc before it is too late.