Madge, Joe, Kiku, and Walt each come to the Metropolitan Museum of Art for different reasons on day in 1941. But then, the four join forces to search for an ancient book of legends about King Arthur hidden somewhere in the museum. Nazi spies are using the book to send one another coded messages about a planned attack on American soil. If the kids find it, they could prevent the disaster and save hundreds of lives. But they don't realize the book has power beyond anyone's imagination . . . and that they're not the only ones determined to claim it.