Since discovering the Canadian Flyer, a magical time-traveling sled, in Emily's attic, Matt and Emily have outrun dinosaurs in the Alberta badlands, evaded slave catchers on the Underground Railroad, and seen the last spike driven into the Canadian Pacific Railway. This time, Emily and Matt land on Citadel Hill in Halifax, Nova Scotia -- just as the SS Mont Blanc, a ship laden with explosives, explodes in the harbor! In the chaos that follows, the two friends find a small dog wandering around with Tim, a dazed young soldier who has just returned from fighting overseas in World War I. Can Emily and Matt help Tim find his niece, Carolyn, the dog's owner? What if she's been hurt in the blast and needs help?