There seems to be no end to the struggles that Isabel must endure during the course of this story. For starters, she's a slave, and she had an opportunity of freedom snatched away from her when a promise made by her former owner, Miss Mary Finch, was not met. Miss Finch had promised Isabel and her sister freedom when she died but unfortunately did not put this in writing, so her nephew sold them.
Their problems seem to go from bad to worse when the sisters are sold to Anne and Elihu Lockton. Anne is a vile character, who insists on being called "Madam" and treats the sisters appallingly. Even in the face of extreme hardship and abuse, Isabel remains steadfast in her goal to protect her sister, Ruth, no matter what.
Later, she is persuaded by the Rebels to provide them with any information that she can glean about Lockton's political activities. In exchange for information, Isabel is promised assistance in escaping when the time is right.
The various challenges that Isabel faces make her courageous and more determined than ever to gain her freedom. She once again takes on the role of spy later in the novel when her friend Curzon has been imprisoned, and her bravery grows. The once-subdued slave winds up stealing a boat and, together with Curzon, rowing to relative safety in New Jersey.
In a nutshell, the various challenges that Isabel faces make her stronger, braver, and more determined.