[1] Julian, Dick, and Anne's mother is ill with scarlet fever, so they, George, and Timmy return to Kirrin Cottage for the Christmas holidays.

Uncle Quentin, who is working on a secret theory in his study, takes a break to hire a tutor, Mr. Roland, to help Julian and Dick catch up with schoolwork they missed while sick.

George is also required to attend the lessons, as she has just spent her first term at Gaylands boarding school and is behind her age level.

When searching a cavity in a wall, Dick finds an old book of medical treatment recipes and a linen map inscribed with Latin words.

The children take the map back to Kirrin Cottage, where Julian guesses that it shows a "secret way" but he is unable to decipher the other words.