Our story begins with an unknown cat returning to the lake after being absent for quite some time. While heading toward a shady spot, the traveler is confronted by an angry tom who asks if the traveler is a Clan cat. The traveler says that it may have been a Clan cat long ago, but things have changed and it no longer wishes to walk that path. After the tom is satisfied that the traveler won't steal its prey, it leaves to return to its home. As night falls, the traveler declares that with their return, the cats of the lake will know its vengeance.
Meanwhile, in ThunderClan, nearly a moon has passed since Jayfeather, Dovewing, Squirrelflight, and Foxleap visited the Tribe of Rushing Water and selected a new Stoneteller. The Clans seem to finally be at peace, albeit an uneasy one. While Jayfeather is pondering the new prophecy he has received about a fourth cat, Bumblestripe tells him that Dovewing has been troubled with nightmares ever since she got back from the mountains. Briarlight also talks with Jayfeather, informing him that they're running low on marigold, due to using up a lot of it on injuries Dovewing received in the mountains. Fortunately, Ivypool is happy to help Jayfeather find herbs. Much to Jayfeather's dismay, the marigold patch has been eaten by a rabbit. Luckily, Ivypool finds some marigold in a tree. She suggests using fox dung to ward off any animal who wants to eat it.
After returning to the camp, Ivypool's father Birchfall tells her that he saw her in the Dark Forest training with Hawkfrost. Ivypool is horrified that her own father has joined the Dark Forest, and grudgingly heads out with him to find fox dung. After finding the dung, they head to the stream by the WindClan border so that they can wash off. After cleaning themselves, Ivypool and Birchfall are attacked by a WindClan patrol which contains Breezepelt. They nearly come to blows, but are rescued by Sorreltail, Bumblestripe, Hazeltail, and Berrynose. On the other side of the territory, Dovewing runs into Tigerheart, who has a warning for her. Dawnpelt believes that Jayfeather is responsible for the death of Flametail, and she is plotting revenge against him. Dovewing dismisses this warning and heads back to the patrol that she was separated from.
Back at the camp, Lionblaze is returning from a training session with Blossomfall when Ivypool returns. The Clan is worried by the news of her confrontation with the WindClan cats, but decides that it's nothing to get worked up about. Afterwards, Purdy tells Dovewing that his moss is really dry and he can't sleep comfortably in it. Graystripe tells his kits to get Purdy some moss, but Firestar has a better idea. He makes Cherrykit and Molekit into apprentices. Cherrypaw gets Foxleap as her mentor and Molepaw gets Rosepetal. When the ceremony is finished, Lionblaze is shocked by the news that Dovewing's power has vanished. He also uses his super deduction skills to figure out that Birchfall is training in the Dark Forest. Later, Cherrypaw and Molepaw are attacked by a fox, but an unknown cat saves them. Jayfeather peeks into Molepaw's memory, but doesn't manage to see the cat. He tells Lionblaze about it, and the brothers wonder if the mystery cat is their missing sister Hollyleaf.
At a Gathering, Onestar lashes out at the ThunderClan cats, claiming that they are stealing prey from WindClan territory. When questioned about it, he explains that there is an intruder in WindClan, but his warriors haven't managed to find out who it is. Tigerheart also warns Dovewing of Dawnpelt's scheme yet again, but is brushed off a second time. The next day, due to the evidence of a cat hanging around the territory, Lionblaze receives permission from Firestar to track down the mystery cat. He manages to find the mystery cat's base, but is dismayed to learn that it is not Hollyleaf, but Sol. Sol remains calm while interrogated by Lionblaze, and asks if he can visit ThunderClan's camp. When he arrives, the cats at first treat him with suspicion, except for Poppyfrost, who asks if it was him who saved her kits. Sol reveals that it was him, then quickly turns the situation to his favor, allowing him to say in ThunderClan as an honoured guest.
The next day, Bumblestripe asks Dovewing to go on a walk with him. Dovewing shoots him down, but is reprimanded by Cinderheart, who tells Dovewing that Bumblestripe really likes her. Dovewing goes back with Bumblestripe, and they enjoy a romantic stroll together. While this is happening, Dovewing's powers begin to come back. After the walk, Lionblaze uses his super deduction skills again and figures out that Dovewing has her power back.
Back in the camp, Yellowfang visits Jayfeather's dreams and tells him that ThunderClan needs more medicine cats if it is to survive the upcoming war with the Dark Forest. Jayfeather realizes that she wants him to awaken Cinderheart's memories of being Cinderpelt, and agrees to do it for her. After awakening, Jayfeather learns that Sorreltail is pregnant again. He summons Cinderheart to a glade where he plans to awaken her memories, but Leafpool appears to confront him. She reveals that Spottedleaf warned her of Yellowfang's plans, and tells Jayfeather that he better not awaken Cinderheart's memories. When Cinderheart arrives, a conflicted Jayfeather tells her that the business he had with her could wait until another day.
In the Dark Forest, Ivypool asks Hawkfrost if Sol is working with them, but Hawkfrost skips around the question and sends her off to do more training. Ivypool notes that Antpelt has gotten crueler since he died. When she awakens, she asks Firestar why Sol is allowed to stay in the ThunderClan camp. Firestar tells her that he doesn't want Sol around either, but if there's a chance to keep their enemies close they might as well take it. Meanwhile, Lionblaze is on patrol when he runs into Onestar. Onestar tells him to kick Sol out of ThunderClan's camp, but Lionblaze says that Sol hasn't done anything wrong yet. However, this is a mistake, as Lionblaze realizes that ThunderClan must ally itself with Sol or seem weak.
Later, Dovewing awakens and discovers with her senses that Sorreltail is kitting somewhere out in the forest. She informs Jayfeather and they go to deliver the kits. Sorreltail has two she-kits, which she names Seedkit and Lilykit. On the way back to camp, Dovewing runs into Bumblestripe, who says that she's amazing for saving Sorreltail. When she gets back to her den with Ivypool, Dovewing sees Sol manipulating Rosepetal, Blossomfall, Hazeltail and Mousewhisker, convincing them that there should be a battle with WindClan. Dovewing tells Ivypool, who remembers the tunnels. Dovewing and Ivypool sneak into the tunnels, where they find Sol telling some WindClan cats to attack ThunderClan. The WindClan cats hear the sisters in the tunnels and head out to investigate. Dovewing and Ivypool flee, but can't find an exit. Suddenly, a newcomer appears and offers to help them escape. Ivypool asks how they know that the newcomer is trustworthy. In response, the newcomer reveals that she is actually Hollyleaf, and tells Ivypool that they have met before. Ivypool remembers her leading Ivypool and Blossomfall out of the tunnels and tells Dovewing that Hollyleaf is trustworthy. The trio escape the tunnels, and Hollyleaf bids them farewell. Dovewing says that Hollyleaf should stay, but Hollyleaf says that she can't go back to ThunderClan. Suddenly, Lionblaze appears and tells Hollyleaf that he won't let her run away again.
Back at the camp, Bumblestripe wakes up Jayfeather and tells him that Dovewing and Ivypool are missing. Suspecting that Sol might have abducted them, Jayfeather awakens the camp and tells them what happened. The cats all run off to find the apprentices. Lionblaze returns with Dovewing and Ivypool, as well as Hollyleaf. The ThunderClan cats are shocked to learn that not only is Hollyleaf alive, but also that Lionblaze managed to track her down. After everyone is up to speed on the developments with Sol, they decide to have a nap. Jayfeather, Lionblaze and Hollyleaf go out for a walk, in which Hollyleaf is informed of recent developments among the Clans. Then Jayfeather runs off to the Moonpool. He visits StarClan, where he meets Bluestar. Jayfeather explains that he wants to talk with Ashfur and get his blessing for Hollyleaf's return, but Bluestar sends him off with a long winded lecture about justice instead.
When day breaks, the ThunderClan cats continue to ponder Hollyleaf's return. They first wonder if she is connected to Sol, but then Thornclaw uses his own super deduction powers to pinpoint Hollyleaf as Ashfur's killer. Hollyleaf is about to confess, but Brambleclaw claims that he saw Ashfur sneak up on Hollyleaf and try to kill her, but epically fail and manage to kill himself instead. The Clan accepts Brambleclaw's lie, and when they find out about Ashfur's crimes, they declare that he got what he deserved. After this bit of drama, Dovewing flirts with Bumblestripe some more. She realizes that while she can't love Tigerheart, she can love Bumblestripe.
Jayfeather is heading back from the Moonpool when he meets up with Mousefur, who asks him about the coming darkness. Jayfeather wonders if Mousefur is the fourth, but dismisses the thought. When he returns to the camp, Jayfeather runs into Lionblaze, who complains that Cinderheart broke up with him because of the prophecy. Jayfeather is tired of Cinderheart's behavior, so he travels into her dreams to set things right. He meets up with her and shows her the life of Cinderpelt. Cinderheart realizes her true identity and regains her memories of her past life. She asks Jayfeather who she is, and he replies that is Cinderheart, and to thank StarClan for giving her another chance.
While Jayfeather is exploring Cinderheart's dreams, Ivypool is yet again in the Dark Forest. Brokenstar calls a meeting, and tells the warriors that soon they will have a chance to prove themselves. Ivypool notes that he isn't being very specific. After Brokenstar gives his speech, Ivypool asks Hawkfrost to take her to see him. Hawkfrost reluctantly obliges and leads Ivypool to Brokenstar. Brokenstar asks Ivypool what she wants, and Ivypool tells him that she wants to be a warrior, whatever it takes. Brokenstar smiles, and tells Ghost Antpelt to fight to the death with Ivypool. Ivypool manages to defeat him and become an official Dark Forest warrior.
Later, Mousewhisker and Cherrypaw eat are ill from trying to treat themselves with herbs and eating the wrong one. Cinderheart manages to save them, and in the process reveals herself as Cinderpelt's reincarnation. Afterwards, Firestar decides that his Clan should train for the battle with WindClan. Hollyleaf takes some warriors down to the tunnels and shows them tunnel tactics, making everyone happy. Then Hollyleaf goes on a patrol where she runs into Dawnpelt, who is still mad at Jayfeather. Dawnpelt throws her weight around with empty threats, and the two part ways. Also, Lionblaze forgives Squirrelflight.
At a Gathering, Dawnpelt accuses Jayfeather of killing Flametail, and the other Clans decide to believe her. Jayfeather flees the Gathering, and meets up with Yellowfang, who is mad at him for reawakening Cinderheart's memories. Jayfeather points out that Yellowfang told him too in the first place, then uses his lecture powers to make her see the errors of her ways. Then he tells Cinderheart to fulfill her purpose in life as a she-cat and have kits with Lionblaze, or there will be be vague doom in the future.
Later, WindClan cats attack ThunderClan. Hollyleaf and Dovewing decide to defeat Sol. They find Sol in the tunnels, where he reveals he once knew another Clan that wouldn't allow him to become a warrior. Hollyleaf defeats him, causing him to flee the tunnels. Dovewing tells Hollyleaf that it's time to go back to their Clanmates. They leave the tunnels together.
Tropes appearing in this book:
- Deader Than Dead: Antpelt's spirit in the Dark Forest disappears entirely after Ivypool strikes a blow that would have been fatal in life.
- Deadly Graduation: The trainees of the Dark Forest are forced to fight to the death as their final assessment.
- Ensemble Cast: The Omen of the Stars arc features four main protagonists: Jayfeather, Lionblaze, Dovewing, and Ivypool.
- The Faceless: The prologue features a mysterious cat declaring that they will have their vengeance on the Clans. It's never explicitly stated who this cat is, but it's almost certainly Sol, based on his role in the book.
- Gender Bender: Furzepaw is called male in one scene:"Yeah, we'll get you then as well!" Furzepaw added, his paws tearing up the grass as he crouched for a pounce.
- HeelFace Return: This happens to Hollyleaf. When last seen in Sunrise, she tried to murder her mother and confessed another murder she'd committed. She returns in The Forgotten Warrior as a friendly cat who helps defeat real villain Sol and a potential candidate for the fourth cat in the prophecy. A novella, Hollyleaf's Story, was later released to explain this change.
- Inferred Survival: Fans had strong suspicions that Hollyleaf survived because of the way her "death" was presented - we "see" it from the POV of a blind character who merely hears rocks caving in and automatically assumes that No-One Could Have Survived That without even trying to dig her out. The characters keep using the word "lost" instead of "died". When the other characters finally realize in this book that she might still be alive since they Never Found the Body, they finally dig through the rubble... and find nothing, so they know she must be out there somewhere. She does return later in the book.
- I Surrender, Suckers: Lampshaded when Antpelt uses it on Ivypool, then expresses exasperation at how she fell for a "tired old trick".
- Never Found the Body: Hollyleaf. No one bothered to try digging up her body after the tunnels collapsed, and - as Jayfeather and Lionblaze realize when they finally do try to retrieve her body in this book - it turns out that she actually survived.
- Posthumous Sibling: Sorreltail's kits Molepaw and Honeyfern died before Lilykit and Seedkit are born.
- Series Continuity Error: In the Allegiances, Troutstream and Rushtail are still listed as apprentices, even though they became warriors in the last book.
- Shipper on Deck: Whitewing, Ivypool and Cinderheart all start shipping Dovewing with Bumblestripe.
- Stranger in a Familiar Land: Hollyleaf, when she returns after a year of having been thought of as dead, and then is instantly revealed to her Clanmates as a murderer (though her family covers for her, claiming Ashfur attacked her.)
- Unwitting Pawn: Sol shows up in ThunderClan for a visit and Firestar decides to let him stay for a bit, despite his evil acts. Later, when Lionblaze is on patrol, Onestar, leader of WindClan shows up and tells the ThunderClan cats to drive out Sol, which they had already been planning to do. However, since Onestar told them to do it, doing so would make ThunderClan look subserviant to WindClan and weaken it, so they are forced to ally with Sol against Onestar rather than driving Sol out.
- Villainous Breakdown: Sol snaps and declares that all the Clan cats will kill each other for reasons that aren't worth more than a mousetail.
- Welcome Back, Traitor: Hollyleaf returns to the Clan after having murdered Ashfur, gone into hiding for over a year, and experienced a change of heart about her birth mother and adopted mother. Upon her return her family covers for her - Brambleclaw claims that he saw Ashfur attack her, and Jayfeather, Lionblaze, and Leafpool, who know the truth, keep quiet - and the Clan welcomes her back pretty much with no questions asked.