Don Morgan is sixteen and ready to earn his keep. His family expects him to take a job at the local fish cannery but Don thinks that if he spends the winter up in Starbuck Valley trapping marten, he could make enough money with the furs to buy his own salmon troller. With his friend Tubby Miller, Don goes up into the wild, unexplored valley of the Shifting River to set out his trapline. The boys' cabin is gloomy, the trapping is poor and Tubby's pleasant disposition begins to sour. Things don't improve when Don begins to cultivate the friendship of their nearest neighbour, a notorious character named Lee Jetson. Lee knows how to trap marten and that is important to Don - he is willing to take chances to ensure the success of the winter's venture in spite of Tubby's obvious and irritating alarm.