London, 1886: Verloc (Toby Jones) runs a seedy sex shop in the heart of Soho. Unknown to his loyal wife Winnie (Vicky McClure) Verloc is paid by the Russian embassy to spy on a dangerous anarchist cell. Summoned to the Embassy, he is given a mission: orchestrate a bombing that will be blamed on the anarchists and provoke a crackdown by the British. Verloc must source a bomb, but hide his actions from Winnie and Chief Inspector Heat (Stephen Graham) of Scotland Yard’s Special Crimes Division. Unable to persuade his anarchist comrades to help, Verloc sets his sights on Winnie’s younger brother Stevie as his accomplice ...

Having recruited Winnie's younger brother Stevie to his bomb plot, Verloc takes him to Michaelis's cottage in Kent to prepare. But while Michaelis's suspicions threaten to scupper Verloc's plans, the Professor makes his own move on Chief Inspector Heat. Back in Soho, Verloc’s wife Winnie waits patiently at home for her husband and brother to return. She is not alone, however, for Ossipon takes the opportunity to make his advances.

As the authorities struggle to make sense of the plot behind the Greenwich attack, Chief Inspector Heat moves to arrest Verloc. Winnie, devastated by the bomb outrage, confronts Verloc and flees into the arms of Ossipon. The pair go on the run, taking a midnight train to Southampton. But the police, led by Chief Inspector Heat, are in hot pursuit ...