After a violent attack threatened both the Wild and Leashed Packs, Lucky knows that the dogs' only hope for survival is to unite. The Wild Pack's ruthless Alpha reluctantly agrees to let Bella and the Leashed Dogs join them--but after learning of Lucky's role as a spy, Alpha casts him out and orders him to never return.

Lucky tries to be happy to have his freedom again, but the forest and ruined city are treacherous for a Lone Dog. Food and clean water are scarce, and a poisonous black cloud is spreading across the sky. For the first time since the Big Growl, Lucky is desperate to have other dogs by his side. But it might be too late. . . .