Captain Athelstan King is a secret agent for the British Raj at the beginning of the First World War.

Heavily influenced both by Mundy's own unsuccessful career in India and by his interest in theosophy, it describes King's adventures among the (mostly Muslim) tribes of the north with the mystical woman adventuress, princess Yasmini and the Turkish mullah Muhammed Anim.

Like Greenmantle by John Buchan, also first published in 1916, it deals with the possibility that Turkey might try to stir Muslims into a jihad against the British Empire.

What was to be Mundy's third novel was originally serialised in Everybody's Magazine in nine parts from May 1916 illustrated by Joseph Clement Coll.

The first film adaptation was The Black Watch (UK title King of the Khyber Rifles), released in 1929 and starring Victor McLaglen and Myrna Loy.