It's a cold winter and Molly is set on discovering a hot springs Grandpa Nat once found in a blizzard, so she sets off with Grandpa, Dad, and Tooey on two snowmobiles and with a list of vague directions. Then, Tooey's hero, dog musher Eugene Pike, is recreating a historic mail run across interior Alaska. When ice breakup at the river threatens Pike's progress to Qyah, it's up to Tooey and Molly to help him finish the mail run.