Mycroft, who has connections in the highest levels of the government, has just received a strange package: An oilskin-wrapped packet containing the papers of a missing English spy named Kimball O'Hara.

Mary quickly realises that this is the same Kimball who served as the inspiration for the famous Rudyard Kipling novel, an orphaned English boy turned loose in India, whose cunning he used to spy for the Crown.

When Russell learns of that Holmes knew Kim thirty years before, she knows the die is cast: she will accompany her husband to India to search for the missing operative.

En route, they encounter the insufferable Tom Goodheart—a wealthy young American who has embraced Communism—traveling with his mother and sister to visit his maharaja friend, Jumalpandra ("Jimmy"), an impossibly rich and charming ruler of the (fictional) Indian state of Khanpur.

With some local intelligence supplied by Geoffrey Nesbit, an English agent taught by Kim, and accompanied by Bindra, a resourceful orphan, the couple travel incognito as native magicians.