Jill and Bob Lawrence are on holiday in Switzerland with their daughter Betty. Jill, a skilled shot, enters a clay-pigeon shooting contest, but is defeated by a German, Ramon, after Betty has distracted her. Later, Jill dances and flirts playfully with Louis Bernard, a friend she has met at the resort, when a shot fired through the window hits him and he collapses. Before he dies, he gives Jill the key to his room, telling her to retrieve something there and take it to the British Consul. Bob searches the room, and finds a message concealed in the handle of a shaving brush, which reads, "Wapping, G Barbor make contact A. Hall, March 21st". Bob leaves the room just before the arrival of the concierge. In the corridor, he encounters Ramon, who threatens him, but withdraws when the concierge appears. Bob and Jill are detained in the manager's office for questioning, but while Bob is waiting, he receives a message, revealing that Betty has been kidnapped and advising them to keep silent.
Back in London, the Lawrences are approached by police, who suspect that Betty is missing. Bob, however, denies it, and the policemen depart, leaving one man behind, who identifies himself as Gibson, from the Foreign Office. He reveals that Louis Bernard was an agent, who had uncovered a plot to assassinate a foreign diplomat on British soil. Gibson questions Bob about the message, but when they receive a warning telephone call from Betty's kidnappers, the Lawrences refuse to co-operate.
Gibson traces the call to Wapping, and when he leaves, Bob, remembering the message, resolves to follow the lead, and sets off for Wapping with his friend Clive. Avoiding the plain clothes police posted at the docks by Gibson, they come across a dentist called George Barbor, and enter; if asked, they will claim to be sailors on shore leave. Clive offers himself for treatment while Bob looks around and, when Clive returns, Bob feigns toothache himself. In the surgery, the dentist sees through Bob's story and Bob overpowers him with gas. Disguising himself in the dentist's coat, Bob witnesses the arrival of Ramon and overhears his conversation with Abbott, who he recognises from Switzerland. Bob learns that they are part of the gang which is holding Betty.
Bob and Clive follow the two men to the Tabernacle of the Rising Sun, where they enter in the middle of what appears to be a spiritual meeting of some obscure sect. They are soon identified as strangers, and Clive is hypnotised, while Bob is prevented from leaving when Abbott and Ramon appear. In the struggle that follows, Bob spots in Ramon's pocket a ticket for a concert at the Royal Albert Hall and, realising that this is to be the location for the assassination, he tells Clive to call Jill and tell her to go to the Albert Hall. Bob is subdued, but Clive escapes and telephones Jill, who sets off before the gang are able to warn her off. Abbott has Bob taken to a soundproofed room above the chapel, where he is reunited with Betty. Clive returns with the police, but Abbott convinces them that Clive is a trouble maker and they escort him away.
Abbott runs through Ramon's instructions: he is to shoot at the climactic moment of the music, when his shot will be drowned out by the sound of the cymbals. Ramon sets off for the concert, and in the lobby meets Jill, giving her a brooch of Betty's as a warning. Back in the chapel, Bob can only listen to the concert on the radio. In the auditorium, Jill begins to realise what is about to happen. When she sees Ramon's gun, she screams just before the shot is fired; the foreign dignitory is hit, but not fatally. Ramon makes his escape, but he is pursued by Jill with the police, and unwittingly leads them to the gang's dockland hideout.
The police surround the building, and a seige begins. In the confusion, Bob manages to get Betty out of her locked room but, attempting to reach the roof, he is shot and wounded by Ramon. Betty reaches the roof with Ramon in pursuit, witnessed by Jill and police on the street below. The police marksman is unable to get a clear shot, but Jill grabs his gun and successfully shoots Ramon down. The police storm the house, and Abbott, by now the gang's sole survivor, is killed. With the siege ended, Bob, Jill and Betty are reunited.
