Set largely in Morocco, the story follows the adventures of Paul Ravenel as he seeks to atone for the disgrace that still attaches to his family name due to the actions of his father who many years earlier in British India had been court-martialled after leaving a key hill fort unguarded.

His mother died when he was young and he had a distant relationship with his father, never understanding why he had renounced his own country, taken his wife's name and nationality, and cut himself off completely from his British friends.

Determined to atone for his father's crime, Ravenel seeks a commission in the French army and is sent to Morocco where he meets and falls in love with Marguerite Lambert, a penniless English woman who – although from a good family – has been forced to earn her keep as a dancer at a disreputable establishment in Casablanca.

Some two years later, de Montignac is ordered to take a battalion into the sacred and closed city of Mulai Idris, to force it at last to open its gates to the world.

Ravenel is to infiltrate an enemy camp and relay its changing position to his French commanders so that aeroplanes - frightening devices never before seen by the soldiers - can be brought in to bomb it.