The Lost Warrior opens with narration from Graystripe, a warrior who was separated from his Clan, ThunderClan, after being kidnapped by humans trying to deforest his home. He is then taken in as a house cat by a Twoleg (human) family. He somewhat likes the Twolegs and their kits but he cannot stand to be away from his Clan and his fellow warriors. He makes an attempt to flee but gets lost in Twolegplace and battles with a kittypet named Duke. After being forced to flee the fight, Graystripe is led back to the nest he has been staying in by a female kittypet named Millie he meets, who assures him that losing to Duke is nothing to be ashamed of. The two cats get to know each other better and Millie finds a small forest in the middle of the Twolegplace. She then shows it to Graystripe and asks him to teach her how to hunt and fight after learning of his previous life. After a dream in which he is visited by his deceased mate, Silverstream, and his daughter Feathertail, and another fight with Duke and his allies, Graystripe finally makes the decision to try to return to ThunderClan. In another dream about Silverstream, after Graystripe tells Silverstream that he wishes he could be with her, Silverstream reminds him that his place is with ThunderClan. She also tells him that he already has a traveling companion. Later, Graystripe asks Millie to come with him to ThunderClan, and is taken aback at her refusal. He then leaves for ThunderClan alone. Graystripe ends up getting lost in Twolegplace for days before collapsing from exhaustion. Millie changes her mind and goes out to catch up to Graystripe. Upon meeting him, she discovers him feverish and weak; he even calls her by Silverstream's name. Millie nurses him back to health and asks him about Silverstream. Then the two set off to try to find ThunderClan. What they don't realize is that ThunderClan, along with the rest of the Clans, no longer resides in the forest.