Darkness Within is the fourth book in the seventh Warrior Cats series, The Broken Code.
In the prologue, StarClan talks about how they shouldn't have trusted a cat to go somewhere. Leafpool is distressed to see that StarClan's connection to the Clans is still blocked, and she fears that without the Clans, they'll all fade away forever.
Picking up where the last book left off, Squirrelflight reveals that she thinks she knows who the impostor is: Ashfur. In order to be sure, she comes up with a plan to trick him into revealing himself, and is successful: he confirms that he's Ashfur. The Clans hold a vigil for all the cats who died in the battle at the end of the last book, and a ghost kittypet tells Rootspring that the recently-dead Clan cats seem to be fading; he wonders if they'll just disappear since they can’t get to StarClan.
Things get tense in ThunderClan. Thornclaw and Lionblaze get into an argument about the leadership of the Clan, and several of the cats decide that they'll go on a "wander" and decide whether to return to the Clan or not. Meanwhile in ShadowClan, Mothwing thinks that Shadowsight needs more training because he’d only become a full medicine cat due to "healing" Bramblestar, and he's assigned to care exclusively for Ashfur as practice. Ashfur tries to convince Shadowsight that the two of them aren’t so different.
Due to the whole fading-spirits thing and the fact that they don't know if Bramblestar's real spirit is still out there, the Clan leaders decide to send a patrol to find the Sisters, who know more about ghosts than the Clan cats. Rootspring, his sister Needleclaw, Bristlefrost, and Spotfur are chosen to go.
Shadowsight's still upset about being treated as if he's no longer a medicine cat. Ashfur is delusional and thinks that Squirrelflight actually wants to be with him, and he says that if she were to go away with him, he'd leave everyone else alone. Shadowsight goes on a hunting patrol, but doesn't do well and feels left out. The medicine cats go to the Moonpool and discuss whether they can still be Clans and appoint new medicine cats without their connection to StarClan. Ashfur taunts Lionblaze into attacking him, and the cats argue about whether they should let Bramblestar's body die or not.
On their quest to find the Sisters, Rootspring, Bristlefrost, and their companions have been pointed in the right direction by a couple of kittypets. They make their way through a Twolegplace, and then encounter Leaf, a tom who had dispersed from the Sisters. Leaf reveals that Spotfur is expecting kits (which seems to upset her, though she later comes to accept it). He says that he can locate the Sisters, and performs a ritual that supposedly allows him to talk to the land. They follow his directions and meet up with them, and the Sisters agree to come with them and do what they can. On the way back, one of the Sisters gets her paw caught in a trap, and they suggest that Rootspring try to use the tom-from-the-Sisters power to commune with the earth and figure out what to do; although he has doubts, he tries it, and successfully figures out what's holding the trap closed.
The patrol and the Sisters make their way to the lake, and the leaders are all fetched to be present at the ceremony. The ceremony works... but something's wrong. All of the summoned ghosts appear to be in pain, screaming, with some of them looking as if they want to hurt the gathered cats. The Sisters send them away, but before they're gone, Rootspring notices Ashfur's spirit watching everything with satisfaction. They realize that this must mean that Ashfur can leave Bramblestar's body at will. The Sisters suggest that the Clans leave the lake due to the angry spirits (who can apparently take physical form when affected by bad energy), and when they refuse, they decide to make a temporary camp nearby so that they can assist the Clans. Sunrise, Rootspring's aunt, gives him some tips about summoning spirits, advises him not to let Bristlefrost go, and promises that the Sisters won't be far away. Rootspring and Bristlefrost talk, and both of them say that they can't imagine leaving their Clans.
Ashfur is confronted about his leaving Bramblestar's body, but he doesn't tell them anything helpful and just mocks them. Tigerstar and Hawkwing are only stopped by killing him when Shadowsight blocks their way, pointing out that he was ordered to take care of Bramblestar’s body and that as a medicine cat he has to help.
ThunderClan's still in a turbulent state and they don't entirely recognize Squirrelflight as leader, and at this point they think that they should try to kill Bramblestar's body to get rid of Ashfur, especially since Bramblestar didn't show up during the summoning ceremony. Squirrelflight accepts their decision and says that she'll ask the other leaders what they want to do, and that if they decide to kill him, she will abide by their decision. They hold a Gathering, and vote to kill him; Squirrelflight runs off ahead while everyone leaves. Rootspring and Bristlefrost assume that she's heading to the ShadowClan camp and run to catch up with her. As they suspected, she's trying to save Bramblestar's body, but they point out that ThunderClan needs her and that if she tries to help him escape, they'll never forgive her and the Clan might fall apart. Squirrelflight reluctantly agrees that ThunderClan must come before her love for her mate.
Shadowsight's watching Ashfur as they realize the Clans are on their way to kill him. Ashfur asks Shadowsight if he knows how much power he's gained, saying that no cat's spirit is safe: suddenly, Shadowsight sees Spiresight's eyes looking out from Bramblestar's face, and then Bramblestar himself: they say that Ashfur is holding them hostage, and Bramblestar tells Shadowsight that if his body dies, there'll be no way to get him back. The leaders arrive (with Lionblaze standing in for Squirrelflight since she doesn't want to take part in killing her mate's body). Shadowsight goes out to speak to them and says "Before you do anything, there's something you should know. Ashfur isn't..." when suddenly Ashfur slips out of his prison and runs off.
Rootspring hopes there's something he can do for Squirrelflight, and, knowing that she wanted to be somewhere she could be close to the real Bramblestar's spirit, he heads to the Moonpool to find her. As he arrives, Ashfur appears next to Squirrelflight and says that he's giving her one last chance: she rejects him, and then he attacks her. Rootspring jumps in to defend her, but Ashfur stuns him with a hard blow. While recovering, he sees Ashfur dragging Squirrelflight into the Moonpool, and they both go under. He goes after them, swimming in the water, but they're nowhere to be found. The book ends ominously: Ashfur was alive, and he'd taken Squirrelflight to a place where only the dead could reach her.
Tropes appearing in this book:
- Cat Concerto: The Sisters perform a ceremony to celebrate Spotfur's pregnancy; they all purr and then yowl to the sky.
- Cliffhanger: The book ends with Ashfur dragging Squirrelflight into the Moonpool and vanishing, evidently into the Dark Forest.
- Divided We Fall: Squirrelflight gives a speech to ThunderClan about how they shouldn't be fighting each other, because they're connected by their love for ThunderClan, while the real enemy is Ashfur.
- A Dog Named "Cat": Some of the Sisters are named after animals: Hawk, Sparrow, and Squirrel.
- Edible Theme Naming: Rootspring and Bristlefrost's patrol attempt to find a kittypet named Pancakes. They don't find him, but they do find his kits - who are named Bacon and Eggs. Another kittypet ghost they encounter is named Cheddar.
- The Exile: Harelight and Icewing have been exiled from RiverClan for fighting against their own Clanmates in the previous book - even though it turns out that they were on the correct side, since their Clan had been supporting an evil spirit that had been attempting to spread discord in the Clans. Mothwing has also voluntarily exiled herself in solidarity with them.
- Grand Theft Me: Ashfur's spirit has control of Bramblestar's body.
- I'll Kill You!: Ashfur taunts Lionblaze, which gets him worked up enough to tell Ashfur that he's going to kill him, and to attack him.
- Immediate Sequel: This book picks up exactly where the previous one ended: with Squirrelflight saying, "I know who has taken over Bramblestar's body! And, if I'm right, it's even worse than we thought."
- Master Actor: Shadowsight is sickened by Squirrelflight's apparent delight to speak to the impostor, and he has to remind himself that she's acting in order to get the impostor to reveal himself.
- The Reveal: After leaving her knowledge of the impostor a Cliffhanger at the end of the last book, Squirrelflight reveals the impostor to be Ashfur. She tricks him into confirming it.
- Screw This, I'm Outta Here: Several ThunderClan cats have become disillusioned with Clan life and their current leadership, and decide to leave: whether temporarily or permanently, they're not yet sure.
- Someone to Remember Him By: Spotfur's mate Stemleaf died in the last book, but she discovers in this one that she's carrying his kits.
- Star-Crossed Lovers: Rootspring and Bristlefrost are from different Clans, and are technically forbidden to be in a relationship. They both love their Clans too much to consider joining each other's.
- Summoning Ritual: The Sisters perform a ritual to summon the spirits of the Clans' recently-lost cats. The ritual itself works, but there's something else going on with the spirits: they look twisted and agonized and angry - and the terrified cats quickly banish them again.
- Surprise Pregnancy: Spotfur doesn't know she's pregnant until Leaf notices and tells her.
- Switching P.O.V.: Shadowsight, Bristlefrost, and Rootspring.
- Trapped on the Astral Plane:
- Ever since Bramblestar lost his life in Lost Stars, he is unable to return to his body since Ashfur is possessing it.
- Clan cats who have recently died seem to be stuck as possibly-fading ghosts, since they can't get to StarClan.
