Cannons at the Gate opens with a secret British mission to collect French gold as reparations just as Napoleon escapes Elba in 1815. When the cargo of gold is seized from the British ship returning to England, the vessel's purser, Ryan Boyle, is charged with piracy. Changing his name to Howard Moorhead with the aid of an uncle, he flees to New Orleans where he becomes a cotton merchant, marries a widow, and builds a plantation in St. Francisville. The novel turns to their son, Percy Moorhead, who leaves college to join the American army in Mexico. As we travel with him on the campaign from Vera Cruz to the Mexican capital, we experience the dramatic and dangerous adventures, the battles and executions, that change Percy from a naive recruit to a young man determined to make his mark on the world and to learn of his father's hidden past. The novel's climax is the occupation of Mexico City when American cannons open the city at Belen Gate.