The book begins with Fireheart and Graystripe performing a traditional silent vigil after their promotion to warrior status in Into the Wild. As their first task as warriors, Fireheart and Graystripe are assigned to find and bring back WindClan, which had been driven out by ShadowClan, and bring them home. They succeed, finding WindClan under a tangle of Thunderpaths (probably a highway interchange), and bring them home.
Soon after, Bluestar apprentices Cinderpaw to Fireheart, and Brackenpaw to Graystripe. During a hunting patrol, Graystripe almost drowns after chasing a vole onto thin ice, falling in the river separating RiverClan and ThunderClan, but is rescued by Silverstream, a RiverClan warrior. Graystripe and Silverstream fall in love; however, this is against the warrior code, a code of honor that all warriors must follow. Fireheart discovers their relationship, and unsuccessfully attempts to discourage them from seeing each other. Graystripe finally agrees to meet Silverstream only at the full moon at the Gathering, when the four Clans meet to share news. Fireheart, later realizes that Graystripe has not been keeping his promise. Because Graystripe is often gone to see Silverstream, Fireheart takes over training of Brackenpaw, as well as training his own apprentice.
Tigerclaw, in his ambition to become leader, sets a trap for Bluestar by the Thunderpath (road), intending to kill her, thus bringing him closer to becoming leader. Instead, Cinderpaw is crippled by the trap. Her leg is broken, and when it heals, she has a permanent limp, preventing her from becoming a warrior. She then trains under Yellowfang to become a medicine cat.
Fireheart is reunited with his sister, Princess, a kittypet living in a Twolegplace (a human town). Princess gives Fireheart her oldest kit, Cloudkit, to take into the Clan as a new apprentice. Although Fireheart agrees to accept the kit, his Clanmates, with the exception of Frostfur and Graystripe, are reluctant to accept him because of his kittypet blood. Bluestar allows him to stay, and Brindleface becomes his foster mother.
Brokenstar the former ShadowClan leader eventually attacks ThunderClan along with several other exiled ShadowClan warriors. After the battle the rogues are driven off with the exception of Brokenstar himself who is blinded by Yellowfang and is kept as a prisoner.
Later, ThunderClan becomes involved in a fight against RiverClan and ShadowClan over sheltering Brokenstar, and WindClan allies themselves with ThunderClan. Graystripe only attacks ShadowClan warriors, afraid to hurt RiverClan cats in case they were friends of Silverstream. Silverstream mistakenly attacks Fireheart but releases him; he attacks her but sees Graystripe's look of dismay, and releases her. Darkstripe witnesses the event and reports it to Tigerclaw. When Fireheart is later attacked by Leopardfur, the RiverClan deputy, Tigerclaw watches as Leopardfur and Fireheart fight, and does not attempt to help Fireheart. Consequently, Fireheart becomes certain that Tigerclaw is not to be trusted.