In 1809, working undercover for the Admiralty's Secret Department, Captain Nathaniel Drinkwater goes ashore and mingles with the dregs of London's dockland. Disguised as a Merchant Marine shipmaster of low character, he encounters a known French agent in a brothel and advertises his plans to carry military cargo to Russia.

Intended to drive a wedge between Napoleon and his new ally, Czar Alexander, Drinkwater's mission is disrupted by tempestuous weather and a lost escort — a gun-brig commanded by his friend and protege. Reaching the British-occupied island of Helgoland, Drinkwater takes great personal risk by resolving on a bold course of action that is not endorsed by the Admiralty.