Lucky is a golden-haired mutt with a nose for survival. He has always been a Lone Dog, relying on his instincts to get by. Other dogs have Packs, but Lucky stands alone.

Then the Big Growl strikes. Suddenly, the ground is split wide open. The Trap House is destroyed. And all the longpaws have disappeared.

Now Lucky is trapped in a strange and desolate new world with no food, foul water, and enemies at every turn. He falls in with others left behind, including his littermate Bella, a Leashed Dog. Relying on other dogs—and having them depend on him—brings new dangers that Lucky isn't prepared for, but he may not be able to survive on his own. Can Lucky ever be a true Pack Dog?