In June 1804, at the glittering debut of Beethoven's Third Symphony in Vienna, a Spanish diplomat meets with Captain Thomas Grey, agent of His Majesty's Secret Service. In exchange for a gigantic bribe, the Spaniard discloses Spain's darkest secret the actual terms of the Treaty of San Ildefonso with France.

Spain's neutrality in Napoleon's war on Britain is only a ruse to keep the British navy from attacking the great treasure-armada now gathering in South America. Spanish warships will depart Montevideo, Uruguay, carrying 2,000 tons of gold; when the gold is safely in Madrid, Spain will declare war on Britain and ally with France to divide the British Empire between them. Britain's only hope is to sink or capture the treasure fleet, and the responsibility of delivering that blow falls to Grey. As Jack Aubrey would have said in such a crisis, "There is not a moment to be lost!"