A young boy's amateur excavation project unexpectedly triggers a complex web of international intrigue and historical secrets. Friday Callendar, the eldest son of the Callendar family, focuses his summer on digging a tunnel through the chalk cliffs behind their home, Marsh Manor. His singular obsession draws him into a dangerous collision with the eccentric Lord Sprockett, a mysterious mineral discovery on the Mediterranean island of Capria, and the arrival of a new tutor, Robin Fawcett. The narrative, told with a distinct mid-century British sensibility, follows the children as they navigate these adult-driven conspiracies while attempting to maintain their own autonomy.

This work is often regarded as a quintessential example of mid-century British children's adventure literature, noted for its blend of domestic charm and genuine suspense. Readers frequently highlight the author's ability to balance the whimsical nature of the Callendar family with the more serious, darker undertones of the mystery. The narrative style is often praised for its wit and the authentic, albeit spirited, portrayal of its young protagonists. It remains a notable title for those interested in the evolution of the mystery genre for younger audiences, maintaining a distinct place in the canon of classic British adventure stories.