A Scotland Yard undercover detective is on the trail of a saboteur who is part of a plot to set off a bomb in London. But when the detective's cover is blown, the plot begins to unravel.
Mr. Verloc is part of a gang of foreign saboteurs operating out of London. He manages a small cinema with his wife and her teenage brother as a cover, but they know nothing of his secret. Scotland Yard assign an undercover detective to work at the shop next to the cinema in order to observe the gang.
Having recently moved to London from the States, the Verlocs are having troubles making ends meet in owning and operating the Bijou, a movie cinema. What Mrs. Verloc is unaware of is that mild-mannered Mr. Verloc, who she considers the kindest man in the world, has gotten himself involved with terrorists who want to cause general havoc in London. Mr. Verloc's first act to prove his worth, sabotaging the electricity grid to cause a general blackout, doesn't quite have the effect that the terrorists were hoping. As such, they decide on a more explosive second act, Mr. Verloc to play his small part in delivering the bomb to the site, namely Piccadilly Circus. What neither of the Verlocs are aware of is that Ted, the young assistant to the greengrocer located next door to the Bijou, is really undercover Sergeant Ted Spencer of Scotland Yard, he who has been monitoring the Verlocs' movements in suspecting they, he or she are involved with the terrorists, the Yard which has also been monitoring the pet store where the bomb is being manufactured. What happens in this situation is affected by Ted falling for Mrs. Verloc who he comes to believe is innocent of any wrongdoing, and the distracted mind of Stevie, Mrs. Verloc's teenage brother who lives with them.
Mr. Verloc, a cinema owner, is part of a gang of saboteurs in London. He lives with his wife, Sylvya, and her young brother, Stevie. They know nothing about Verloc's secret. Scotland Yard assigns an undercover detective, Ted, to work in a shop near the cinema and investigate the man. The head of the gang assigns Verloc to put a bomb in the metro. The man sends Stevie there with the "bag".
- First we see a dictionary page highlighting SABOTAGE, then a power station with one chimney smoking in the moonlight, then a gaudily lit London street at night. The lights in the streets go out. Lit only by flashlight, we see a large machine half in water. Several hands excitedly fuss with the machinery, digging up a muddy substance. A first voice says: "Sand!", then another voice "Sabotage!", and a third : "Who did it?" The face of Karl Verloc (Oscar Homolka), walking in the shadows, emerges to fill the screen.
Customers at the BIJOU cinema exit to the street after the power goes off. Some loudly insist on getting a refund. At the box office, the clerk and the owner's wife are hesitant to refund anything, as they are short of cash.
A youngish man Ted Spencer (John Loder) stands on a step and spouts off comically as though he knows the law and repeats there is no need to return money, as there is no fault by the cinemas owners.
Meanwhile, Verloc comes into the BIJOU surreptitiously although Ted observes him. He quietly goes to bed at the dwelling in back of the screen and lies down as if he had been napping.
Verloc's young wife Sylvia (Sylvia Sidney) is hesitant to give refunds, and believes her husband is still out, but Ted tells her he saw him come in, so she goes into the apartment, finds Verloc, and asks whether to refund the money. Verloc tells her to go ahead and give refunds, as he will be getting some money shortly.
Just as refunds are about to be given, the power comes back on, customers laugh and reenter the BIJOU.
The Verlocs are the owners of the BIJOU. They live, together with her little brother Stevie (Desmond Tester), in the rear apartment. Right after the blackout, the cook leaves, leaving Stevie in charge of finalizing dinner. Stevie fumbles and breaks a dish, which he hides in a drawer.
Scotland Yard suspects Verloc is an agent for a foreign power, part of a gang planning other attacks. Detective Sergeant Ted Spencer was assigned to investigate Verloc under cover, and attempt to get hard evidence against him. Spencer poses as a greengrocer's helper, selling fruit and vegetables in a shop right next to the cinema. The family comes into the shop often, and he ingratiates himself with Stevie and his sister by providing many extra services.
Ted is attracted to the beautiful Sylvia but she wants nothing to do with his advances.
Ted invites Stevie and Sylvia to a full lunch at Simpson's, a fashionable restaurant. At the lunch, Sylvia explains that her husband is a good man who has been kind to her and her little brother.
She is an American who came to London with Stevie to escape the Depression in America, but when the economy crashed in England, too, they were down on their luck and to survive she married a kind older man who protected them. They are running the BIJOU, but struggling economically.
Verloc receives a phone call and goes to the London aquarium to receive pay and more instructions. He is tailed by police, who want to identify the entire saboteur gang, but knowing the foreign bosses are "people you and I will never catch" as one policeman tells another.
Verloc meets his contact in front of an illuminated tank where a huge turtle is swimming. A passerby (Charles Hawtrey) enthusiastically describes the reproduction of fish at the aquarium and compares them to humans.
Verloc is paid but scolded because his power station stunt was disappointing. "London must not laugh" is the order given to Verloc. "We want the English to worry about what happens in England and thus pay no attention to what happens elsewhere in Europe". Agents must not fear retribution to earn their money.
Then the contact assigns Verloc to put a bomb in Piccadilly Circus station "the center of the world", and produce fear, not laughter. Verloc isn't a forceful villain, just a weak, man of no moral fiber who objects to the idea of murder but decides to go through with it for money.
The bomb is to be prepared by an agent who runs a pet shop as a front. Verloc realizes that he is in over his head, and tries to get out of the fearsome responsibilities, but he is scolded again and he accepts. In a dreamy sequence the scared Verloc sees the fish swimming in the tank and imagines them as motor cars filled with passengers moving around Piccadilly Circus when an explosion demolishes all buildings around. Suddenly the fish in the tank reappear.
Again tailed by Scotland Yard, he proceeds to the pet shop. At the pet shop, an old biddy wants to return a canary that won't sing. The shop owner Professor Chatman (William Dewhurst), has no trouble getting the bird to warble, implying that it's the lady's fault. Verloc is led to a stuffy kitchen where a chaotic nconventional family of three, a father, daughter, and granddaughter live, to talk to Chatman.
Chatman opens a cabinet, reaching for bomb making stuff and first finds the granddaughter's doll in the cabinet. He then explains to Verloc that the bomb is up to Chatman, only the delivery will be up to Verloc.
They agree to disguise the bomb in a cage with canaries, addressed to Stevie, who will enjoy the birds.
A clandestine meeting for gang members is arranged for Verloc's apartment. Ted has no difficulty identifying something is afoot, since anyone wanting to see Verloc must walk up to the front of the Bijou, pass into the dark screening room, walk along its side to a curtain next to the screen. Behind the curtain is Verloc's door.
Ted gets Stevie to show him how to get behind the screen to a window from which he might overhear what is going on at the meeting Verloc is having with other shifty-eyed foreigners in ill-fitting suits. Unfortunately, he is spotted by the conspirators and pulled into the room.
The conspirators end their meeting, telling Verloc the job is his alone. Verloc exits to talk to his neighbor the greengrocer, and elicits comments that confirm that Ted is a policeman under cover.
Meantime Sylvia has become gradually more suspicious about Verloc's activities. However, she hesitates and delays.
Professor Chatman sends Stevie a pair of canaries in a cage, with the package bomb concealed in a drawer underneath. Verloc reads a sealed message that the bomb is set to go off at 1:45. Ted admits he is a detective and detains Verloc asking him details over and over, pretending he has no suspicions of Verloc himself. Verloc, wanting an alibi for the time of the explosion, arranges for Stevie to take the package to Picadilly and leave it in the station cloakroom by 1:30. The excuse is that a film canister needs to be returned nearby. It must be walked over, because it is inflammable and it cant be carried on a bus.
The heartbreakingly cheerful brother is carrying the bomb along with film canisters labeled "Bartholomew the Strangler. Because the Lord Mayor's parade is on that day, Stevie is delayed by all manner of events. A street vendor pulls him out of a crowd in order to demonstrate a new toothpaste. He's stopped from crossing a street because the parade is marching by. He enjoys watching the parade. The parade has snarled traffic. When he gets on a bus the conductor ironically warns, "Don't set fire to me or the other passengers." The bus keeps hitting red lights.
There are many cuts between the oblivious Stevie, with a time bomb in a package under one arm while petting a fluffy puppy with his free hand and talking to an old lady, those big outdoor clocks whose arms get ever closer to 1:45, until the explosion.
Ted receives orders to go to the explosion scene. When Stevie does not get back home for a meal, Sylvia starts to worry and ask questions, and learns the awful truth from Verloc.
Verloc confesses his part in the sabotage, while standing near the model boat which she and her brother had made, then blames Spencer for Stevie's death, saying he prevented Verloc from delivering the bomb himself. Verloc suggests to Sylvia that they could have a child of their own, as if to compensate for her loss.
Sylvia, deeply depressed and confused at the revelation, hears laughter from the audience. She wanders into the theater in shock, and sits to watch "Who Killed Cock Robin," a 1935 Disney cartoon featuring a parody of Mae West. Momentarily, she laughs, but when the arrow is loosed and slays poor Cock Robin, the laughter on her face changes to an expression of agony and terror.
Her eyes and expressions subtly convey an emotional cascade of stunned realization, immense sadness, and suppressed hysteria. She realizes what a monster her husband is.
Sylvia explodes after sitting to dinner with Verloc. He orders his still shocked and dazed wife to a "get a grip" and get prepared to deny everything. He goes on to order her to scold the cook about the overcooked cabbage soup on the dinner table.
Without dialogue, the camera shifts from her face, to Verloc's, to the food they are eating, to her hold on a carving knife, until they both rise, stare at each other, and it happens: she stabs him in the stomach with the knife.
Meantime, Ted discovers, at the scene of the blown up bus, part of the film canister from the BIJOU that most likely was carried by Stevie, so he returns to the BIJOU.
We see Sylvia sitting, dazed, right past the doorway, and in the foreground, the feet of the slain Verloc.
When Spencer arrives at the apartment, he realizes what has happened, but insists that Sylvia shouldn't admit that she knifed her husband. He takes her out of the apartment and tries to convince her that he is in love with her, and talks about getting on a boat crossing the Channel to France and together escape the police.
Professor Chatman's daughter berates him for leaving a bird-cage with the shop's identification at Verloc's, and insists he go back to retrieve it. Chatman enters the apartment just ahead of a squad of detectives coming for Verloc. When the detectives knock, he locks the door and announces he has a bomb and will blow it up if they enter.
The detectives at the door assume they are hearing Verlocs voice, and take the threat seriously. They evacuate the building. They argue about who should stay near the apartment: the single guy says he will stay and let the married ones go to a safer place, but one of the marrieds jokes that since hes already married, his life is over anyway so he might as well stay.
Sylvia is dazedly responding to Ted's love declaration, and kisses him, but she refuses to run away, and is just on the edge of confessing to a policeman when an explosion and fire at the cinema intervenes. The evidence of her crime is destroyed, and effectively prevents the policeman from remembering whether it was before or after the explosion that she said, "My husband is dead!"
At the end we see Sylvia and Ted walk away together.