Mr Norman is returning from a business trip. He passes a hitchhiker who is having engine trouble and offers him a lift to the nearby train station. The hitcher is taken to Richmond railway station but accidentally leaves behind a diamond necklace. Armed with an address left by the hitcher, Mr Norman tries to return the necklace the following day. Norman meets a woman - possibly his mistress - and gives her the necklace, but they are interrupted in their embrace by the leader of a diamond smuggling ring. He blackmails Mr Norman into using his yacht to illegally transport the diamonds.

Some time later, Mr Norman's wife worries about her husband. He denies that anything is wrong, but Mrs Norman's suspicions are aroused and she asks her son to spy on him. The couple's eldest daughter can also see that her mother is worried and she persuades her younger sister and her sister's boyfriend to spy on their father as well. The next evening, Mr Norman's son follows him to the smuggler's hideout, but doesn't act yet.

Several days later, the daughters and fiancé decide to go to the hideout, where the father is being pressured by the smugglers to take diamonds over to Amsterdam on his yacht. The spying daughters interrupt the smugglers, but are overpowered and tied up while the father is led away to his yacht. The mother and eldest son arrive some time later and rescue the strung-up kids before rushing to the yacht. They arrive at the mooring only to find the yacht gone and the father left for dead on the riverbank.

Later, the newly reunited family enjoy breakfast together. The morning paper reveals that the yacht has been wrecked in a storm and the smugglers killed.