Royal Navy Lieutenant St. Vincent Halfhyde, Captain Watkiss, and Admiral Daintree are under orders to round Cape Horn on a voyage from Chile to Britain to deliver traitor Sir Russell Savory to his fate. Their old enemy, Vice-Admiral von Merkatz, is on their trail in hopes of freeing Savory. But when a revolutionary coup jeopardizes the safety of the British ambassador to Uruguay, the flagship Halcyon and its crew are diverted to effect a rescue. Rightly supposing that the British attention will be focused on the ambassador rather than on their captive traitor, von Merkatz steams confidently up the River Plate and, with extravagant promises, quickly makes allies of the new Uruguayan regime and enlists their help in recovering Savory. When Watkiss misreads a chart and makes three of his ships collide, the British are left ill-prepared to contest the challenge from von Merkatz and his German squadron.