In this Book, Firestar is now the leader of ThunderClan and receives his nine lives where Bluestar tells him the prophecy "Four will become two, Lion and Tiger will meet in battle and blood will rule the forest." He chooses his deputy to be Whitestorm, an experienced warrior who is quite old. Then Tigerstar attempts to unify all four Clans claiming that it would help the Clans survive. While Leopardstar agrees, Tallstar and Firestar both refuse to join this alliance, which Tigerstar has called "TigerClan". Accompanied by Ravenpaw, who is on a visit to ThunderClan, Graystripe and Firestar go to RiverClan and find that Graystripe's kits, Stormpaw and Featherpaw, and Bluestar's kits, Mistyfoot and Stonefur, being held prisoner. Since they are all half-ThunderClan and half-RiverClan, Tigerstar accuses them of being half-Clan cats and therefore, in his opinion, traitors. Firestar, Graystripe, and Ravenpaw manage to rescue Mistyfoot, Stormpaw, and Featherpaw, but Stonefur is killed protecting Featherpaw and Stormpaw.

In an attempt to convince Tallstar and Firestar to join his alliance, Tigerstar reveals to them BloodClan, a vicious group of rogues in the nearby town. When both leaders still refuse, Tigerstar then orders BloodClan to fight for him, but they do not do so. Scourge tells Tigerstar that he is the only cat in charge of BloodClan, and that after Firestar tells everyone Tigerstar's bloodthirsty history and that he cannot be trusted to divide power, he has decided that there will be no battle. Tigerstar attacks Scourge, who kills Tigerstar easily, ending all nine of Tigerstar's lives with one blow, by cutting him open, from throat to tail. Scourge then gives all of the forest Clans three days to leave; otherwise, they will have to fight BloodClan for the forest. To face this danger, the four Clans unite, forming an alliance which is known as "LionClan". The battle is won when Firestar kills Scourge, though he loses one of his own nine lives in the process. Without its leader, BloodClan scatters. With the forest returned to normal, the four Clans become independent once more.