Seventeen-year-old Teresa thought she knew Philadelphia, but she stumbles into Gloria's Gateway Books & Records and discovers a portal to parallel universe where Napoleon conquered Europe and Great Britain and where dragons still live. She also discovers Tom, whose Philadelphia is the capital of a Britain exiled from the Home Islands, now a part of Napoleon's l'Empire.

This Philadelphia boasts electric carriages (invented by Sir Benjamin Franklin), as well as airships for long-distance travel. However, Tom's father believes dragons hold the secret to designing heavier-than-air craft—but dragons have almost died out. When his father is kidnapped, Tom and Teresa travel by airship to the subjugated Home Islands in search of answers. There they discover who kidnapped Tom's father, and how to save the dragons!