"Clan cats, cats who lived beside the lake during their waking moments, surrounded him. [Jayfeather] could feel their warm hearts beating, quickened by the lies of the dead. There was no hiding now from what was happening. Cats from every Clan were being trained by Dark Forest warriors to rise against their Clanmates, trained to break every part of the code the Clans had long fought to protect."
Soon after the events of Power of Three, new points of view are introduced in Dovewing and Ivypool, kin of Firestar, and the first point of view outside of ThunderClan, Flametail of ShadowClan. Erin Hunter has said that this series will bring back old important characters. 
The third cat is revealed as Dovepaw, who has the power of super sight and hearing. Lionblaze, Jayfeather, and Dovepaw soon discover that the Dark Forest is training cats in their dreams. A battle between the Clans and the Dark Forest is imminent, and Lionblaze and Jayfeather must find the fourth cat before it’s too late. In the Great Battle, the fourth cat is revealed to be Firestar, who ends Tigerstar’s tyranny once and for all by killing his spirit form. However, Firestar dies of his wounds soon after.
Lionblaze and Jayfeather must figure out who the third cat is before it is too late. Dovepaw and Ivypaw, two she-cats born to Whitewing and Birchfall, become prime candidates for the third cat, as they are related to Firestar through Cloudtail. Dovepaw develops super hearing and sight, and is revealed as the third cat. Ivypaw becomes jealous of Dovepaw. Suddenly, a drought hits the Clans, and the lake all but completely dries up. Lionblaze, Dovepaw, Rippletail, Petalfur, Toadfoot, Tigerheart, Whitetail and Sedgewhisker are sent to bring back the water. With help from some kittypets, they succeed in destroying a dam built by beavers, bringing back the water.
The Dark Forest continues to train cats in their dreams. Dovepaw’s relationship with Tigerheart continues to grow, and while her sister is distracted, Ivypaw is lured to the Dark Forest by Hawkfrost. A tree falls into the ThunderClancamp, killing Longtail and permanently injuring Briarpaw. Ivypaw is then told by Tigerstar to convince Firestar to attack ShadowClan, and Lionblaze accidentally kills Russetfur, the ShadowClandeputy, in the battle.
Flametail, a ShadowClan medicine cat, starts receiving visions that warn him of something terrible coming. Tigerheart and Dovepaw continue to meet in secret, and Lionblaze and Cinderheart get serious with their relationship. However, when Cinderheart finds out about Lionblaze’s power, she thinks they can never be together because of his destiny. Ivypaw continues to train in the Dark Forest, but is eventually convinced by Jayfeather and Lionblaze to spy on the Dark Forest for the Clans. While out on the ice of the frozen lake, Flametail falls through and drowns. Jayfeather tries to save him, but everyone ends up thinking that he killed Flametail, especially Dawnpelt.
Jayfeather and Lionblaze discover that Hollyleaf is still alive. Soon after, Jayfeather has visions about the Tribe of Rushing Water, and Firestar allows him to go to the mountains with Foxleap, Squirrelflight, and recently made warrior Dovewing. Meanwhile, Ivypool continues to spy on the Dark Forest for the Clans. While in the mountains with the Tribe, Jayfeather begins having strange experiences where he travels back in time as Jay's Wing. While in the ancient times, he falls in love with Half Moon and ultimately makes the decision for the the Ancients to leave the lake and live in the mountains. Back in modern times, Jayfeather chooses Stoneteller‘s successor, and the ThunderClan cats leave the mountains.
When Sol returns to the lake, he convinces Onestar to attack ThunderClan using the tunnels. Hollyleaf is forced to reveal herself to help ThunderClan defeat WindClan. She lets Sol escape due to her honor of the warrior code. Meanwhile, the Dark Forest continues to train living cats in their dreams, and seem to be close to launching a massive attack. Cinderheart learns about her past life as Cinderpelt, and Dovewing and Bumblestripe begin to grow close.
The Dark Forest finally launches their attack on the living Clans. Ivypaw convinces most of the the cats who trained with the Dark Forest to fight with their Clanmates, as most of them were lied to and didn’t know about the Dark Forest’s plans. StarClan comes down to help the living cats battle, but there are still many deaths, including Spottedleaf, Ferncloud, Mousefur and Hollyleaf. Firestar is revealed as the fourth cat. In an epic final battle, Firestar kills Tigerstar’s spirit form, but sacrifices his last life in the process. Bramblestar becomes the new leader of ThunderClan, and appoints Squirrelflight as his deputy.
Lionblaze and Jayfeather discover that Hollyleaf is still alive. Jayfeather and Dovewing go to the Tribe of Rushing Water, where Jayfeather has time-travel experiences as Jay’s Wing, and makes the decision for the Ancients to leave the lake. The ThunderClan cats choose Stoneteller’s successor and leave.
The Dark Forest attacks the living Clans. The Clans defeat them with the help of StarClan, but many cats, spirit form or otherwise, are killed. Firestar is revealed as the fourth cat, and kills Tigerstar’s spirit, sacrificing himself in the process. Bramblestar becomes the new leader of ThunderClan.
Omen of the Stars (EN), HarperCollins (paperback box set), 25 September 2012
Omen of the Stars (EN), HarperCollins (paperback box set; reprint), 3 November 2015