This adventure begins when Pete catches a large fish with a mysterious tag on its tail. Then the children’s unhappy friend, Bobby Reed, disappears in a leaky old boat on the Muskong River, and the whole family helps in the search. Uncle Russ lends them his boat, the Sweetie Pie, and they all set off on an exciting trip down the river.

Gram and Gramp Hollister invite the Hollister children to visit Québec for Mardi Gras and the annual Winter Carnival. Their visit coincides with the strange disappearance of a master sleigh maker, and the children set out to find him and experience new adventures in a most interesting and historic location.