When a power cut blacks out much of London, police identify sabotage as the cause. At a local cinema, Sylvia tries to cope with customers demanding their money back in the absence of her husband, proprieter Carl Anton Verloc. Unbeknown to Sylvia, however, Verloc returns home under cover of the darkness. When the customers will not be mollified, Ted, an assistant on a nearby greengrocer's stall, comes to Sylvia's aid. Anxious, Sylvia goes inside, and is surprised to find her husband in. At Verloc's suggestion, she returns to the front office to offer the customers refunds, only to find that Ted has all but succeeded in pacifying the crowd. The two argue, until the power returns and the crowd make their way back into the cinema.
When, later, Ted delivers fresh cabbage to the Verlocs at the request of Sylvia's young brother, Stevie, he mentions seeing Verloc return during the blackout, and is surprised by Verloc's denial. Ted leaves the stall and makes his way to Scotland Yard, where it emerges that he is an undercover detective assigned to observe Verloc. When he reports the night's events, his Superintendent instructs him to try to talk to Sylvia.
The next day, Verloc meets his contact at a Zoo aquarium, where he is given instructions to deliver a bomb to Piccadilly Circus. Verloc is uncomfortable at the loss of life involved but, desparate for money, he reluctantly accepts the assignment. Meanwhile, Ted takes advantage of Verloc's absence to take Sylvia and Stevie to lunch in order to probe her for details about Verloc. He leaves satisfied that whatever Verloc's own involvement in the sabotage, Sylvia is innocent.
On his contact's instructions, Verloc - unaware that he is being followed - makes his way to a pet shop, where he meets the bomb-maker, and arranges for the delivery of a caged bird the coming Saturday. The cage will hold a concealed package containing the bomb, primed to explode at 1.45 that afternoon.
The following day, Ted observes a stranger calling on Verloc, and resolves to try and spy on the meeting. With help from Stevie, who he tells that he is aiming to give Verloc a surprise, Ted makes his way behind the cinema screen to a small window overlooking the Verlocs' living room. There he observes a meeting of conspirators who Verloc has enlisted to carry out the bombing. Before Ted is able to learn anything, however, he is spotted and hauled into the room. He makes his excuses and leaves, but one of the conspirators recognises him as a policeman. Unnerved, the conspirators disperse, leaving Verloc to handle the bombing alone. Anxious, Verloc tells his wife about Ted's true identity, but claims ignorance of his reasons for watching the cinema.
When Saturday arrives, the bird cage is delivered, and Verloc passes it off as a present for Stevie. Ted calls on Sylvia and when she is cold with him, he explains to her the reasons for his observation. Verloc, realising that Ted's presence makes it impossible for him to do so himself, resolves instead to send Stevie to deliver the bomb package, along with some film cans. Verloc tells Stevie to leave the package in a cloakroom at Piccadilly Circus, stressing that he must do so before 1.30, and reminding him that he cannot take film cans on public transport. Stevie leaves, and Ted, with Sylvia behind him, goes to question Verloc.
Stevie makes his way to Piccadilly, but is delayed, first by a salesman, and then by the Lord Mayor's show, during which he loses track of the time. When the parade passes, he realises that he is going to be late, and manages to persuade a bus conductor to let him on a bus bound for Piccadilly, but traffic is heavy, and he is still on board the bus when 1.45 arrives and the package explodes.
When Sylvia reads that a clue has been found at the scene of the explosion, she realises that Stevie is dead and that Verloc is responsible. She confronts Verloc, who is regretful but defensive. Sylvia is tormented by grief and later, at dinner, she is consumed by anger with Verloc and eventually kills him with a carving knife. As she sits in a state of shock, Ted arrives and learns of the murder. He persuades her to escape with him to the continent, believing that the body will not be found until the arrival of the housekeeper in the morning. However, the bomb-maker, aware that his cover may be blown, makes his way to the cinema, with the police on his tail. Sylvia, distraught, tries to confess her crime to the Chief Inspector, but is brushed off. Meanwhile, the bomb-maker runs through the cinema in his desparation to evade the police, and comes across Verloc's body. Cornered, he blows himself up, just as Sylvia finally tells the Chief Inspector her husband is dead. The explosion removes all evidence of the murder, and the confusion of events makes the Chief Inspector uncertain of the timing of her statement in relation to the explosion. With Sylvia in the clear, she and Ted make their getaway.
