Napoleon has been defeated at Waterloo, and the ensuing peace brings with it both the desertion of nearly half of Captain Aubrey's crew and the sudden dimming of Aubrey's career prospects in a peacetime navy. When the Surprise is nearly sunk on her way to South America—where Aubrey and Stephen Maturin are to help Chile assert her independence from Spain—the delay occasioned by repairs reaps a harvest of strange consequences.

The South American expedition is a desperate affair; and in the end Jack's bold initiative to strike at the vastly superior Spanish fleet precipitates a spectacular naval action that will determine both Chile's fate and his own.