Pierre's father cut his thumb, so now he won't be able to work, therefore the family won't have enough money to survive the winter. Pierre goes out in his father's place and enlists to be a voyageur. He leaves his family and his schoolwork behind.
He begins his journey to Grand portage. All the paddling hurts his hands (which is why he's sad) because he hasn't done this before, and it makes them blister and swell up. However, he's still going strong, and can carry his weight when he travels with the packs on his back.