Army doctor George and his gunner friend Harry no longer serve in the British Royal Army. The first left and treats patients in a small London practice. The other received the Victoria Cross for bravery, but when he persuaded a general's daughter to run away with him, it was taken away from him again and dismissed from the army under a different pretext. He became a drunkard losing his last money at cards.
The offer of two thousand pounds promised to them by the Sultan of Morocco seems to be an easy and quick way for them both to get back on their feet. The Sultan needs them to command the maintenance of the three older cannons he acquired for his fight against the rebels led by his brother Bou Hamra.
But their whole enterprise soon becomes unpleasantly complicated. Although they are subjects of the British Queen, they become unintended participants in the war campaign, are captured and tortured. The most dangerous part of their mission begins when they become hostages of a local ruler nicknamed the Lord of Atlas. The man with his face still covered is hiding a secret that will change everything.