Alfred Hitchcock directed this suspense thriller about a couple whose daughter is kidnapped so they won't tell the police about an assassination.
In Switzerland skeet-shooting Jill Lawrence (Edna Best) gives her daughter Betty (Nova Pilbeam) a broach. Jill dances with Louis Bernard (Pierre Fresnay) in front of her husband Bob Lawrence (Leslie Banks), who unravels her knitting by attaching it to a dancer. Louis is shot from the window and while dying tells Jill to get a brush from his room to the British embassy. Bob Lawrence finds a note about G. Barber in a shaving brush; then he gets a note threatening his daughter if they talk to the police. He shows this to his wife, and she faints.
In England Lawrence refuses to help the police, as Gibson from the foreign office says Louis knew of an assassination plot. The Lawrences put their child first, especially after the kidnapper calls and they talk with Betty. Gibson traces the call, and Bob Lawrence goes there looking for Barber. When the dentist Barber tries to put him out with gas, he puts the gas on the dentist. Lawrence and his friend Clive (Hugh Wakefield) go into an occult tabernacle where Clive is invited to blank out his mind. Lawrence is kept from leaving with the uninitiated by a woman holding a gun. Gang leader Abbott (Peter Lorre) tells Lawrence that Barber has apoplexy and invites him into the circle. When Lawrence learns Betty is asleep, he throws furniture; but they refrain from shooting so as not to attract police. Lawrence is put in a room, and Clive calls Jill to tell her to go to the Albert Hall. When Clive brings a policeman to the door, two gang members get Clive arrested. Betty is sadistically brought by Abbott to her father to say good-bye. Abbott plays music so the assassin will know when to shoot.
At the Albert Hall a man gives Jill the broach. During the concert she looks at the balcony. She screams; the dignitary is shot, and a man escapes. Radio reports only a slight wound. A policeman is shot at the door of the hideout, and shooting ensues. Police get rifles from a gun-shop and take over nearby rooms. Lawrence finds his daughter, but he is shot. Betty goes on the roof, followed by the assassin. A policeman hesitates because Betty is in the way; but Jill shoots the man. Abbott, hiding, is shot by the police, and Bob and Jill are reunited with Betty.
Though suspenseful, the gang seems merely evil, since their motives for assassinating are not explained. The shoot-out was based on a recent event in England and showed the British police's unfamiliarity with guns. The wife and daughter are both portrayed as bold and brave.