In London, a sudden power blackout disrupts a cinema owned by Karl Verloc, Oskar Homolka. As patrons demand refunds, Verloc slips through a back entrance to the living quarters above. He washes sand from his hands, and when his wife, Sylvia Sidney as Mrs. Verloc, comes to confront him, he pretends to have been asleep. He instructs her to refund the money, saying he has >some money coming in< anyway. Just as the cash is about to be handed out, the lights snap back on and the moment passes, leaving everyone tense but unaware of the danger looming over the city.
The next day, Verloc meets his contact: a member of a small terrorist cell from an unnamed European country who plans a new wave of attacks in London. The contact is irritated that the newspapers mocked the earlier blackout and orders Verloc to plant a parcel described as "fireworks" at the Piccadilly Circus tube station on Saturday, timed to the Lord Mayor's Show. Verloc is uneasy about killing, but the plan allows someone else to do the dirty work. He is given the address of a bird shop, whose owner also acts as a bomb maker.
Scotland Yard keeps a watchful eye on Verloc. Detective Sergeant Ted Spencer, John Loder, goes undercover as a greengrocer's helper near the cinema. He befriends Mrs. Verloc and her younger brother Stevie, Desmond Tester, who lives with them, by treating them to a meal at Simpson's. By the end of the meal, Spencer is convinced that Mrs. Verloc is innocent and finds himself drawn to her.
Verloc visits the bird shop, where the bomb maker promises to craft a time bomb set to explode at 1:45 p.m. on Saturday. That night, the terrorist group gathers in Verloc's living room above the cinema. Spencer tries to eavesdrop but is spotted and recognized, causing the meeting to break up in a flurry of nervous tension. Verloc tells his wife that the police are investigating him, yet he maintains his innocence.
The following day, Verloc receives a package containing two caged canaries—intended as a gift for Stevie—and the bomb itself. Spencer arrives with Stevie and warns Mrs. Verloc of Scotland Yard's suspicions. Verloc, increasingly on edge, asks Stevie to deliver a film canister to another cinema and, on the way, to deposit another package in the cloakroom at Piccadilly Circus station by 1:30 p.m. He claims it contains projector parts awaiting repair.
Unknowingly carrying the time bomb, Stevie faces a string of delays including the Lord Mayor's Show procession. He eventually boards a bus to Piccadilly Circus, despite the rule against carrying nitrate film on public transport. The bomb explodes on the bus, destroying Stevie and several others.
Verloc confesses to his wife, yet shifts the blame to Scotland Yard and Spencer for Stevie's death. As they sit down to dinner, she grows angry and withdrawn. He draws closer with a knife in hand, and he walks into the blade and dies. Spencer arrives to arrest Verloc, but he realizes the tragedy and urges that she should not feel compelled to testify to stabbing him. He contemplates leaving the country with her.
The bomb maker goes to Verloc's flat to retrieve the birdcage, fearing incriminating evidence. The police are already nearby, and when they arrive, Mrs. Verloc tries to confess, but moments after she says her husband is dead, the bomb maker detonates a bomb he carries, killing himself and destroying Verloc's body.
In the aftermath, a cautious police superintendent remains unsure whether her confession came before or after the explosion, as Ted and Mrs. Verloc walk away together into the crowded city streets.
