The year 1777 is bleak indeed for the cause of American Independence, with the British army twice defeating Washington and taking the capital city of Philadelphia and the Royal Navy sweeping aside the defenses of the Delaware Bay.

For Captain Isaac Biddlecomb and the men of the half-built frigate Falmouth, things are direr still. After managing to slip through a British blockade, they find themselves trapped in a desolate harbour on the New Jersey coast and menaced not by the British but by the outlaw bands that terrorise the countryside and see Falmouth as a potentially valuable prize.

Deserter Angus McGinty steals Biddlecomb's most potent weapon, the captured British sloop Sparrowhawk, leaving him to face the ruthless Pine Robbers on his own, with only his diminished crew and the near-useless local militia to help.

Meanwhile, Virginia Biddlecomb, trapped in occupied Philadelphia, sees her chance to play a clandestine role in the fight. In the course of her activities, however, she lets slip information that will put her husband, his ship and his crew in mortal danger, leading to a desperate race to get the unwieldy Falmouth to a place beyond the reach of the Royal Navy.