The plot of the novel hinges on the actions of a modest technical journalist, Keith Stewart, whose life has been focused on the design and engineering of scale-model machinery. Stewart writes serial articles about how to create scale models in a magazine called the Miniature Mechanic, which are extremely well regarded in the modelling community — as is he.
Keith's sister had married a wealthy naval officer, recently retired from service at the opening of the story. The couple plan a long pleasure cruise in their small yacht before settling in British Columbia, meanwhile leaving their 10-year-old daughter with Keith and his wife. Before leaving, they ask Keith for assistance in hiding a jewelry box in the yacht's concrete ballast. When the couple are killed in a shipwreck in French Polynesia, Keith becomes the permanent guardian and trustee of his niece (hence the title). But, the solicitor handling the estate finds that the money has disappeared.
