The books begins Fireheart is determined to uncover the truth about the Clan deputy, Tigerclaw, whom he believes is untrustworthy, whilst risking the trust of his leader, Bluestar, and even the possibility that she might suspect Fireheart himself as a traitor.

ShadowClan and WindClan eventually lead an invasion of the ThunderClan camp in an attempt to kill Brokentail whom ThunderClan is sheltering but are driven off.

Meanwhile, Fireheart's best friend, Graystripe, risks the trust of his entire Clan as he continues having a forbidden love affair with Silverstream, who is from RiverClan. Fireheart and Graystripe face danger from Tigerclaw, who has noticed Graystripe missing from camp many times. Silverstream eventually dies while giving birth to Graystripe's kits when they come early. Graystripe is left heartbroken. ThunderClan then learns about his forbidden love with Silverstream and feel that they cannot trust Graystripe anymore.

Later, Tigerclaw leads a band of rogues into ThunderClan camp, pretending to defend the Clan. During the battle, Tigerclaw corners Bluestar, the leader, in her den and tries to kill her. Fireheart arrives just in time to save her and drove Tigerclaw away. Bluestar is shaken and shocked by Tigerclaw's mutiny, though she manages to announce Tigerclaw's exile from the Clan. Tigerclaw then asks Darkstripe, Longtail, and even Dustpelt if they want to join him in exile, but each cat refuses. With Bluestar having regained her trust over the cat that had exposed Tigerclaw's treachery, Fireheart is made deputy, although the announcement of the position is made after moonhigh, which is against Clan tradition. As a deputy, life is harder for Fireheart and things change rapidly. Fireheart is unsure whether he would be a good deputy, but he is sure that he has not seen the last of Tigerclaw.

At the end of the book, Graystripe, feeling torn between loyalties, heavy-heartedly decides to join RiverClan in order to raise Silverstream's kits. Fireheart attempts to have him reconsider, but this later proves to be unsuccessful. Fireheart is left heartbroken with the thought of losing his friend with whom he shared many adventures, and was the first Clan cat he had ever met since his days as a kittypet. He believes he will be lonely without his best friend.