When Jacob Wheeler transfers to Millie's private school in England, he laments the thought of living in his cousin's "perfect" shadow. But when Jacob is suspended from school for stealing, Millie is convinced that Jacob has been framed. Granny forces him to volunteer at the Lambeth Palace library, where he learns about a recent rash of rare book thefts from libraries around the world.

As he learns about library marks, Jacob becomes convinced that a seventeenth-century hymnbook he found at the village church is stolen. Jacob safeguards the book until Grandpa returns from a trip to Italy. But when Granny mistakenly delivers the hymnbook into the hands of a mysterious woman, Jacob enlists Millie's help to search for the woman's connection to the book as well as her whereabouts. As they learn about sophisticating books and the extent that book thieves will go to pinch a rare book, Jacob and Millie realize that the woman may be connected to their school, and possibly even a more serious pattern of book theft.

Will Jacob be able to collect enough evidence to locate the stolen hymnbook before it is lost forever? Will he be able to clear his name before he ends up expelled from school?