Sir Russell Savory - whom Halfhyde captured after Savory tried to sell naval secrets to the Germans - is in irons and homeward bound aboard the flagship Halcyon. When the Uruguayans have a revolution, throwing the British ambassador into jail they are diverted to effect a resue.
Misreading a chart Watkiss makes three of his ships collide, leaving a pursuing German squadron free entrance to the River Plate, where they enlist the Uruguayans help in recovering Savory.