Taking place in an alternate history, the story presents the further adventures of Dido Twite, an eleven-year-old Victorian tomboy, aboard a whaling ship and in Nantucket.

The UK editions and their covers were initially illustrated by Pat Marriott, whose moody pen and ink drawings were used for most of the Wolves Chronicles, and several other Joan Aiken books.

Dido Twite awakens aboard the whaling ship The Sarah Casket, where she has been cared for in a coma by Nate Pardon, a young sailor who found her adrift in the Atlantic Ocean after the adventures of Black Hearts in Battersea.

While exploring the surrounding countryside, Dido encounters a mysterious but comic foreigner, who proves to be sighting a gigantic cannon intended to assassinate King James III by blowing up the royal palace; Mr. Slighcarp, the Sarah Casket's first mate, is part of the plot together with his sister Letitia, the villainess of The Wolves of Willoughby Chase.

Disbelieved by the other adults, Dido and Pen, with the help of Nate, attempt to disable the cannon, whose recoil would send the island of Nantucket crashing into New York City.