Time travel continues, and fifth-grader Duck Smith and his best friend Sera Frost face new challenges to save the future world. Directly from the ship "Santa Maria" of Christopher Columbus, they find themselves in medieval Paris on the eve of its siege by fierce Vikings, who surrounded a small fortress on an island in the middle of the Seine with their huge boats. While Duck, Sera and their historian friend Rick frantically try to understand what historical mistake they have to correct, the situation is rapidly deteriorating. As a result, the guys find themselves on opposite sides of the barricades: Dak - in the Viking camp, Sera and Rick - outside the walls of Paris. And they don’t have the slightest idea what to do.