In this Book, When a pack of dogs begins to run wild in the forest, Fireheart, deputy of ThunderClan suspects that Tigerstar, ShadowClan's new leader, is behind it. When Bluestar names Cloudtail a warrior even though all the apprentices had worked hard, two apprentices, Swiftpaw and Brightpaw, try to fight the dog pack themselves in an attempt to prove themselves. However, Swiftpaw is killed and Brightpaw disfigured by the pack of dogs, losing part of her face. While Brightpaw is recovering from the mauling, she repeats words she heard the dogs say: "pack, pack" and "kill, kill".

Normal Clan life resumes, but Fireheart begins to have a strong sense of curiosity of how this happens. To find out, he asks Brightpaw if she remembers what happens, but she can only remember the words "Pack, pack, kill, kill". Whitestorm reports to Fireheart that they smell dog near Snakerocks, so they head there.

When they arrive, they find a trail of dead rabbits is found leading to the camp, ending with Brindleface in front of the camp, who had been killed by Tigerstar. Fireheart orders a patrol to get rid of the rabbits. Brindleface's death shocks everyone, and after her burial, the camp is evacuated to Sunningrocks to protect ThunderClan. With Bluestar still insane, it is up to Fireheart to destroy the pack. He and the senior warriors decide that a patrol of cats will lead the dogs to and over the nearby gorge, drowning the dogs. But, before he leaves he apologises to Sandstorm for not obeying Bluestar and they, end up falling in love, really just realising the love they had for each other. The plan works: each cat successfully leads the pack closer to the gorge. Fireheart, the last in line, keeps a good distance between him and the pack leader, until Tigerstar appears and pins Fireheart down. As the dog leader gets dangerously close, Tigerstar leaps away, leaving Fireheart to be killed by the pack. As Fireheart struggles against the dog, Bluestar, sane once more, slams into the dog, releasing its grip on Fireheart and throwing the dog—and herself—over the side of the gorge.

Fireheart leaps down the cliff to the river and dives in to save Bluestar. Fireheart manages to get a hold on her, but is barely able to keep their heads above the water. Soon, however, Bluestar's RiverClan children, Stonefur and Mistyfoot, discover Fireheart struggling in the river and help drag Bluestar to the bank. There, Bluestar pleads with her kits to forgive her for lying to them about their parents. They forgive her. Bluestar then tells Fireheart that she realised that they were not all traitors to the Clan, like Tigerstar was. She also tells him that he is now the leader of ThunderClan, and that "you are the fire that saved my Clan". With that, she loses her final life.

Fireheart, now Firestar, is filled with grief, and Mistyfoot asks if she and Stonefur can help carry her back to camp and sit vigil for her.