Spring 1750. Knavesmire, outside York. French sailor Black Jack is hanged for a fatal punch thrown during a brawl. His body is taken to Mrs Gorgandy, who sells unclaimed bodies to local physicians. Twelve-year-old draper's apprentice Tolly Pickering sees Jack wake up, having survived hanging by placing a bent spoon in his gullet. Jack forces Tolly to flee with him.

In Beverley, Parson Hall and Dr Jones agree to take Mr and Mrs Carter's 12-year-old daughter Belle into their asylum. Belle has been mentally unstable since a fever at the age of five, and the Carters fear that rumours of hereditary madness will endanger elder daughter Kate's marriage to Lord Somers.

Tolly prevents Jack from attacking passengers in a trapped coach. Jack frees the coach and is rewarded by its passengers. Jack sets a trap, which stops the coach taking Belle to the asylum. Belle escapes. Jack is offered a reward to catch her. Frightened of madness, he sends Tolly. Tolly finds Belle. Belle describes a tower that only she can see, and attacks Tolly for denying its existence. Belle doesn't remember her surname or address. The coach and Jack have gone. Tolly plans to take Belle to his uncle, a sea captain in Hull.

Tolly and Belle reach a travelling fair. Dr Carmody sells an elixir of youth with the aid of a scam involving his associate, Hatch. Carmody invites them to join the travelling fair folk. Jack has also joined them, He plans to return Belle and claim the asylum's reward. Hatch tries to steal Belle's clothes, and leaves after a fight with Tolly, who is given Hatch's job.

Hatch blackmails Hall and Jones, threatening to tell Mr Carter that the asylum has lost Belle. In Beverley, Hatch blackmails Carter, threatening to reveal Belle's illness.

Tolly reads that madness is easier to address if it is not hereditary. Tolly's parents died at sea. Belle's memory improves: she remembers the day she met Tolly. Jack steals money from the travellers and is forced to leave.

Tolly writes to his uncle. Belle tells Tolly she loves him. Tolly says he will marry her when they are at sea. Jack returns, hungry and ill after weeks alone. To the anger of other travellers, Jed allows Jack to stay. Recovering further, Belle now remembers her surname and address.

September. With Kate successfully married, Carter writes to the asylum requesting Belle's return. Jones and Hall are angry that Hatch blackmailed Carter. They pay Hatch to find Belle. Jones races to Carter's house, but Hatch arrives first. Carter has been shot dead. Jones smuggles Hatch out of the house undetected to protect their secrets. Hatch claims that Carter fought off a further blackmail attempt and Carter's gun fired accidentally during a struggle. Jones gives Hatch a job at the asylum to keep him out of trouble.

The travellers reach Beverley. Carmody visits the Carters. He tells Belle that her father died in an accident, but confides in Tolly that many believe it was suicide. Revealing this to Belle, Jack persuades her that her madness is hereditary and that she should enter the asylum for Tolly's sake. She leaves for the asylum in York. Discovering this, Tolly goes to York. The asylum refuses to let him see Belle.

October. Tolly works in a pub to be near Belle. Asylum workers still stop him seeing Belle, and steal his gifts. Tolly's uncle's ship arrives at the local harbour. Jack feels guilty about his treatment of Belle, and he and Jed arrive to help Tolly.

Jack smashes open the asylum door and the three rescue Belle. They discover that Carter didn't commit suicide. Hatch arms an inmate to stop them, but the inmate instead kills Hatch.

Jack and Tolly aim to prove that Carter didn't commit suicide, to prove that Belle's illness isn't hereditary. They dig up Carter's grave: his body is missing. They discover that Mrs Gorgandy sold Carter's body to the local physician. The physician reveals that Carter was murdered, shot in the back.

Jack and Jed leave. Tolly and Belle sail away on Tolly's uncle's ship, which has a tower like the one she has often described and seemingly imagined.