It is the first year of the Revolutionary War, and 10-year-old Isabel (it doesn't say in the book how old Isabel is this is just a guess) and her 5-year-old sister Ruth look forward to freedom, as promised in the will of their now-deceased owner. Unfortunately, they land in the hands of their owner's nephew, who refuses to seek out the will and disbelieves that a slave like Isabel is literate. He sells the sisters to an abusive couple, Elihu and Anne Lockton, who bring them to New York City from their previous hometown of Newport, Rhode Island. Isabel, determined to gain her and her sister's liberty, meets a local slave boy named Curzon, who asks her to spy on the Locktons, who are secretly Loyalists. He wishes to report back any suspicious plans to his own Patriot master (patriots and loyalists are opposing sides).
Isabel overhears her new master Lockton involved in a plot to bribe Patriots to defect, and she tells Curzon. Patriot soldiers soon search the house and arrest Lockton, releasing him the following day and leading to much domestic chaos in the home. Isabel soon also overhears Lockton plotting with the New York mayor to assassinate General George Washington. After Ruth undergoes an epileptic seizure, Isabel runs to the local Patriot fort and exposes the conspiracy. Wanted again by the Patriots, Lockton escapes the city by hiding inside a cheese crate, leaving his wife in charge of the house.
The British fleet enters New York Harbor, the Patriots topple a statue of the British king, and the Patriots formally declare independence. Isabel wakes one morning to find that Ruth has been sold to the West Indies. Furious, she confronts Madam Lockton, who attacks her. Isabel runs out to the Patriot fort, but Madam pursues her. The fort's colonel tells Isabel he cannot help her, and she is imprisoned for violating her master and branded with an "I" on her cheek for "insolence", taking six days to recover. Curzon encourages Isabel to keep spying but she ignores him, believing the Patriots do not care about the issue of slavery. At a gathering of slaves, she learns the British are offering freedom to slaves that join their cause.
The British enjoy a major victory at the Battle of Long Island, and Master Lockton returns home with the occupying British forces, some of whose soldiers begin living inside the Lockton house. Isabel is sent across town to tend on Lockton's sickly but kind aunt, Lady Seymour, when a fire breaks out across much of New York City. Isabel and Lady Seymour barely manage their way back to the Lockton home, where many more British soldiers and their wives are soon housed. Isabel worries about Curzon, whom she discovers has been taken by the British as a prisoner of war, beginning to visit him at Bridewell Prison by bribing the guard with food. A bookstore owner gives her Common Sense, which she reads. She continues tending to Lady Seymour and visiting the Patriot prisoners, sometimes passing them military secrets. Madam finally finds out about her activities, hits her, and confines her within the house, revealing that Ruth has not actually been sold yet and is being held in Charleston, South Carolina.
Isabel escapes, grabbing a map and filling out a pass to declare herself a freedwoman. She returns to Bridewell to liberate Curzon where she finds him extremely weak, falsely announces that he is dead, and sneaks him out of the prison in a wheelbarrow. She steals a rowboat and they cross the New Jersey border, where she asks him if he is ready to begin the long journey on foot to Charleston.
