What happens when Harvey and Perry decide to ski back to their home in Sawmill Landing after attending a race in Grand Marais in the novel, Northern Lights?

After attending a race in Grand Marais, Harvey and Perry decide to ski back to their home in Sawmill Landing. Several days into the journey, they become lost. They spend weeks wandering through the Arrowhead, finding meagre shelter in the remains of an old logging cabin and in a rundown shed.