The dogs making up the Pack are a breed of enormous, strong, muscular guard dogs with short, sleek, black and brown pelts and sharp teeth. They are portrayed as unintelligent, possessing only instincts related to eating and killing, and having a reduced vocabulary, mostly barking "Pack, pack" and "Kill, kill". They were released into the wild digging a hole underneath their pen.
The Pack Leader is the lead dog; all other dogs follow him. He seems to be the most intelligent; for instance, he is the one who realizes that they can escape by digging a hole under the fence. He keeps the Pack together; once he dies, the rest of the pack scatter, without any other dog assuming lead position.
The dogs are brought to the Treecut Place by Twolegs to guard it and keep out pillagers after the fire has devastated the forest. However, they are enticed by the prey smells coming from the forest, and one night they escape after digging a hole under the fence. They set out to hunt prey in ThunderClan territory, and shelter in a cave at Snakerocks.
Soon, a ThunderClan patrol notices scent of dogs and dog dirt near Snakerocks, but Fireheart, the deputy, dismisses the report, assuming that it was only a Twoleg walking a pet dog in the forest. Meanwhile, Cinderpelt, the medicine cat, starts to have dreams about animals roaming the forest and chanting "Pack, pack" and "Kill, kill", but does not know what to make of it. At the same time, three Twolegs arrive and search for the dogs, yowling and poking into hollows; however, they do not find the Pack.
After a couple of days, Fireheart leads another patrol to Snakerocks, where they smell fresh dog scent, and find scraps of prey. Fireheart concludes that these many traces could not be left by a Twoleg pet, and realizes that something is wrong. Whitestorm comes upon more traces of dogs near the RiverClanborder. Fireheart assumes that a single dog is loose in the forest, and that the three Twolegs were searching for it the other day. He says that it should be driven off, as it eats ThunderClan prey, and poses a threat to the cats. He tells the Clan about the dog, and that they should be especially careful. Sandstorm suggests that it could be a dog from the farm beyond WindClan grounds. Bluestar is convinced that the scraps of prey are the result of WindClan hunting on their territory, and the two Clans almost come to battle. Later, she visits the Moonstone, where StarClan reveals that there is evil loose in the forest; a "pack" that will bring death and destruction.
ThunderClan's first contact with the dogs is when Swiftpaw and Brightpaw go to Snakerocks to investigate and possibly drive away whatever is stealing prey, in an attempt to prove that they are worthy to be appointed as warriors. The dogs attack them; Swiftpaw is killed, while Brightpaw is seriously injured. A ThunderClan patrol finds the two apprentices, and Brightpaw mutters about the Pack before losing consciousness. Later, she cannot recall what happened, and the cats still have no idea what kind of evil is menacing them.
Tigerstar, the ShadowClanleader, finds out of the dogs as well, and plans to use them to take revenge on ThunderClan, from where he was exiled. Longtail notices Tigerstar feeding dogs with dead rabbits at Snakerocks, and this is when the ThunderClan cats finally find out that there is actually a whole pack of enormous dogs living on their territory. Cinderpelt realizes that this is the "pack" she had the dreams about.
The same night, Tigerstar lays a trail of dead rabbits leading from Snakerocks to the ThunderClan camp, hoping that the dogs will be lured to the camp. He kills Brindleface and puts her body at the end of the trail, to give the pack a taste for cat blood and slaughter the other cats as well. A patrol notices this and Fireheart warns the Clan, saying that they should leave the camp at once and shelter at Sunningrocks. He forms a plan to get rid of the dogs; once the Pack shows up, several cats would lead them to the gorge, and lure them over the edge.
Fireheart stations the warriors and apprentices who would lead the dogs at different positions, and goes to the steepest part of the gorge himself, to take over the final stretch. The dogs eventually appear, led by the Pack Leader. Fireheart leads them to the gorge, but Tigerstar stops him, holding him down so the dogs can catch up. The Pack Leader grabs Fireheart by the scruff and shakes him, but Bluestar comes to his aid, driving the Pack Leader over the edge, where he drowns. Two other dogs fall into the water as well, and the surviving ones scatter and leave the forest.