Sherlock Holmes, retired and living in Sussex, is drawn back into one final, urgent case during the height of World War I. Sherlock Holmes, now a gentleman farmer, finds his quiet retirement disrupted by the looming threat of war and a complex espionage case involving a German agent. Aided by his loyal companion Dr. Watson, Holmes must navigate a world of deception and international intrigue to prevent vital information from falling into enemy hands. The narrative framework is that of reminiscences, looking back on past cases and this final, significant investigation.

This collection is widely regarded as a significant entry in the Sherlock Holmes canon, particularly for its inclusion of the titular story, which offers a unique wartime setting for the detective. Readers often appreciate the blend of classic deductive reasoning with the heightened stakes of international espionage. The stories are celebrated for their intricate plots and the enduring partnership between Holmes and Watson. As a compilation of later works, it provides a glimpse into Holmes's life and methods as Conan Doyle continued to explore the character's adventures.