This book is the last book of the Omen of the Clans arc, and features many different cats fulfilling their destinies in a final battle against the evil of the dark forest, ending in an epic battle where Firestar and Tigerstar both summoned their clan to fight against the other, where Firestar eventually killed Tigerstar, ending the Dark Leader's rule, and fulfilled the ancient prophecy of that fire will save the clan.
The book start with the darkness of the Dark Forest already looming over the clans; Star clan can not see nor contact the living clans, with many cats from the living clans secretly training in the Dark Forest, following Tigerstar and Brokenstar, plotting against the clans and overthrowing them based on their hatred.
From the last book, the three cats from the kin of your kin prophecies, Jayfeather, Lionblaze, and Dovewing, were told by the Ancients that there is a fourth cat who would save the clan, and the three were searching for that cat. They looked through many clans, seeing different traits of them that might indicate them being the fourth cat. One guess being Ivypool, Dovewing's sister, who can dream her way into the Dark Forest, getting involved in Tigerstar's training, but yet still loyal to her own clan. When that guess was proven wrong by realizing that her ability was pretty normal, they thought about other cats outside the clan, like Mothwing, who is a medicine cat without believing in star clan, getting rid of many troubles for her, that guess was also proven wrong. They even thought about Hollyleaf, their lost sister, but that was also proven wrong since there is no way to know where even is she.
When the three was about to give up, the ancient cat Rock told them to keep looking, telling them that there wouldn't be a dark forest if the three were not born, and they must keep looking for the fourth cat for their clan and themselves.
Very soon after, Mothwing came from the Wind Clan camp and found Jayfeather, Jayfeather thought there were a way to reunite the medicine cats, but Mothwing had something else in mind. Mothwing told Jayfeather that she thinks she saw a sign, which Jayfeather think was highly unlikely since Mothwing don't even believe in Star Clan. When they got there deep in the Wind Clan territory, they saw a reed that is burning on its very end. According to Mothwing, the reed had been burning for days and days now, despite the rain and mist that otherwise would've stopped the fire. Jayfeather instantly recognized the significance of the sign, and rushed to the moon pool. He was able to dream into Star Clan despite of the Barrier between Star Clan and the living clan thanks to his power, and met with Star Clan warriors. Yet, Star Clan is different from before, all the Star Clan warriors are now divided, and it took Jayfeather quite a while to see who he wanted to see, Flametail. Flametail was the Shadow Clan apprentice medicine cat who drowned in the lake, Jayfeather was trying to save him but failed. A Shadow Clan warriros saw this and thought Jayfeather was the one who drowned Flametail, and told this to all cats at a gathering, causing the medicine cats to split up. Now Jayfeather have a way to talk to Flametail directly, asking him to tell the rest of his clan in dream that he isn't killed by Jayfeather, but instead fell in the water himself. It took Jayfeather quite a while to convince Flametail, but eventually persuaded him, and this was Flametail's destiny to bring the medicine cats together again.
After the medicine cats reunited, they all decided to dream to their own clan in Star Clan together, and tell the divided Star Clan to reunite, at the same time, Bluestar told the truth to Jayfeather that the fourth cat is actually Firestar. They did this successfully, and later convinced the living clans to be united as well just like Star Clan, agreeing on sending patrols to eachother's territory to have a lookout for eachother when the Dark Forest attacks. 
The three cats soon told Brambleclaw, and a few other senior warriors about the dark forest and their prophecies, which made Brambleclaw question Ivypool, making Ivypool reveal to him that Blossomfall, Mousewhisker, and Birchfall are training in the Dark Forest. When Leafpool comes in the medicine den, she asks to assist Jayfeather, even if it is just collecting moss, but Jayfeather tells her it would be a waste of her skills and he wants her working with him. Leafpool is shocked and reminds him that she is no longer a medicine cat, but Jayfeather tells her that there is a battle coming and that if they must go against StarClan for the sake of the Clans, then they must. Leafpool apologizes again, and Jayfeather replies that there nothing to forgive. Afterwards, the battle starts. The Dark Forest attacks ShadowClan, and Lionblaze and Graystripe go to help them. The Ancients and Midnight come somewhere in the middle of the battle. During the battle, Littlecloud tells them that Blackstar lost a life, and while they fight, Shredtail pins down Ratscar and prepares to kill him, calling him a traitor, but Snowbird knocks him away. Shredtail then points out to them Redwillow, who is dragging Dawnpelt by her scruff across the clearing. When Redwillow joins Shredtail, Snowbird lunges at him, but Blackstar stops her and in fury, calls him a traitor to his Clan and kills Redwillow. Shredtail then challenges Lionblaze, and Lionblaze ends up killing Shredtail. When the Dark Forest warriors stem to a small amount, Lionblaze and Graystripe return to ThunderClan. Ivypool is searching for Mousewhisker, Blossomfall, and Birchfall after they go missing in the forest. She encounters Tigerheart, unsure which side he is on, the Dark Forest or the Clans, and confesses she is fighting on the side of the Clans. Tigerheart spits at her and says that Dovewing chose the Clans over him, while Ivypool retorts that he should've been loyal to his Clan and doing his duty instead of pursuing her sister. The battle with WindClan and the Dark Forest begins, and Applefur, Mousewhisker, Birchfall, and Blossomfall have changed sides. Ivypool stops Breezepelt from attacking a WindClan tom and Thistleclaw, Snowtuft, and Hawkfrost confront Ivypool, while Tigerheart fights for the Clans. Ivypool is trapped and would've died if Hollyleaf hadn't showed up and knocked Hawkfrost back. After Hawkfrost flees, Ivypool notices Hollyleaf has a wound in her neck and frantically carries her back to ThunderClan with Tigerheart's help. ThunderClan has already defeated a Dark Forest patrol. Dovewing scents fresh blood and Ivypool and Tigerheart enter the camp with Hollyleaf in their jaws. Dovewing suspects that Tigerheart caused the injury to Hollyleaf, who is bleeding badly. Ivypool tells Dovewing that Hollyleaf saved her life. Hollyleaf wakes up for a brief moment, saying that she couldn't bear to go to StarClan without knowing her mother. She forgives Leafpool, saying that she was a good mother, and then bleeds to death afterwards from a throat wound among others. Thistleclaw and the others break through and snarl at them that they lost the WindClan camp because of them. During the battle, Breezepelt attacks Lionblaze, only to be stopped by Ivypool, who asks if he really wants to destroy the Clans for Brokenstar's sake, but Breezepelt reveals that he wasn't doing it for Brokenstar, and he tells Ivypool that he, Hollyleaf, or Jayfeather should have never been born. Breezepelt tells Lionblaze that it's his turn to die, but Crowfeather stops him, and tells him to leave. Breezepelt obeys and flees from the battle. Mapleshade tries to murder Sandstorm, because she had every blessing StarClan stole from Mapleshade. Spottedleaf stops her, but the Dark Warrior reminds Spottedleaf of her stolen love. Spottedleaf gets angry and tells her there was nothing to steal and that Sandstorm made Firestar happy, and Mapleshade slaughters Spottedleaf. Sandstorm chases Mapleshade away, Firestar comes over and begs her not to leave, stating that she said she would wait for him in StarClan, but Spottedleaf apologizes and Yellowfang appears to tell Firestar to let her go, that her destiny was to save Sandstorm, and Spottedleaf fades. After Spottedleaf is killed for a second time, Ferncloud is murdered by Brokenstar, who is then killed by Yellowfang. Brambleclaw comes into camp, bringing Hawkfrost and killing him after he confronts Ivypool. Tigerstar comes forward and congratulates Brambleclaw on a job well done, and the final confrontation between Firestar and Tigerstar is faced. Firestar ends up killing Tigerstar, but in the process Firestar also dies from his wounds. Brambleclaw declares that the battle is over, and that their victory belongs to Firestar. Every cat cheers for Bramblestar, who is now ThunderClan's leader. Bramblestar chooses Squirrelflight as his deputy. Bramblestar says that everything she does is for the best of reasons, and he understands. Leafpool thanks Squirrelflight for everything, and Squirrelflight tells her that she would do it again in a heartbeat. Squirrelflight then accepts her deputy role. Rock and Midnight reiterate the entire prophecy that was originally given to Firestar, "There will be three cats, kin of your kin, with the power of stars in their paws. They will find a fourth, and the battle between light and dark will be won. A new leader will rise from the shadows of his death, and the Clans will survive beyond memories of his memories. This is how it has always been, and how it will always be."