When Timmy comes down with whooping cough, it looked like the Nelson family's long-planned trip to Canada would have to be forgotten. But Mr. Nelson still had to go for business, and Mother suggested that Mark and Kerry go with Father anyway as they could learn a few French phrases in advance and explore Quebec while their father worked. Kerry immediately senses there is a mystery about La Maison des Amis, where they take a room for their stay. Can she and Mark solve the mystery?
