In this Book, Six months have passed since the previous book, The Darkest Hour. Bramblepaw, Tigerstar's son, has received his warrior name, Brambleclaw. Firestar has had two kits with Sandstorm, named Squirrelpaw and Leafpaw. Squirrelpaw is apprenticed to Dustpelt, and Leafpaw is apprenticed to Cinderpelt, to train to become the next medicine cat of ThunderClan. While Leafpaw and Cinderpelt search for herbs, StarClan, the cats' ancestors, sends Cinderpelt an ominous warning in some burning bracken, a picture of a tiger running through fire, which she interprets to mean that fire and tiger will destroy the forest. Cinderpelt concludes that the warning must be about Squirrelpaw and Brambleclaw, the daughter of Firestar and the son of Tigerstar, respectively. They share the warning with Firestar, who later decides to keep Brambleclaw and Squirrelpaw separated.

In a dream, StarClan tells Brambleclaw, Feathertail (Graystripe's daughter), Crowpaw, and Tawnypelt (formerly Tawnypaw, Brambleclaw's sister) to listen to what "midnight" has to say. Eventually, they begin a journey in the direction of the setting sun. Squirrelpaw tags along and Stormfur insists on accompanying them to protect his sister, Feathertail, as the six cats trek into the unknown world. On their journey, they meet an old loner named Purdy who helps the Clan cats get to the sun-drown place (ocean). Eventually, they reach the sun-drown-place and enter a cavern inhabited by a highly intelligent badger known as Midnight, who reveals to them that humans will destroy the forest and that the cats must either leave the forest or die. She also tells them that a dying warrior will lead the Clans to their new home. The book ends with a short epilogue back in the forest, where the humans begin to destroy ThunderClan's territory.