For general tropes for the spin-off, go to Rider Time Ryuki.
A mysterious woman brings 13 people into the Mirror World to restart the Rider War. Among them are Shinji Kido, Ren Akiyama, Jun Shibaura, Goro Yura, Miyuki Tezuka and Takeshi Asakura. These 13 people (aside from Asakura) have no memories about their past. The woman tells them to fight each other, and the winner must be found on the 7th day. The winner will be released from the Mirror World with restored memories. Shinji does not believe the woman, but she transports everyone to a place in the city where Mirror Monsters are running amok, and tells them to begin fighting. Asakura transforms into Kamen Rider Ouja and challenges the others. Holding the Ryuki Advent Deck in his hand, Shinji recalls that he used to transform before, but he cannot remember the full context. He transforms anyway and enters a fight with Ouja.
On the 4th day of the War, Shinji is shown with Tezuka, Ishida and Kimura inside an abandoned house. They are apparently a team, opposed by Jun and his two comrades: Ishihashi and Tozuka. Shinji keeps having a dream about a man in black outfit (implied to be Ren-his old and trusted friend in the original Rider War). As Jun's gang finds Tezuka's team, a huge battle breaks out. However, both sides are forced to retreat when hordes of Mirrors Monsters appear and interrupt their battle. Shinji activates his Final Vent to destroy a group of monsters before leaving with his team.
Later, Shinji, Ishida and Kimura are seen talking with one another. Shinji offers them some drinks, asking them to celebrate their survival of the battle. Kimura and Ishida however, are not interested, and the former is appalled aat Shinji's optimism because of the fact that they are still trapped with no way out. Shinji responds by saying that it is because they are in a dilemma, they must find a way to be happy and enjoy themselves. At this point, he reflects on the events of the first day, and reveals that he does not really trust the mysterious woman's words very much. Kimura tells him that believing in the woman is their only hope at the time. Shinji says he has been fighting all this time so that he can get back to the real world and find the man who keeps appearing in his dreams (implied to be Ren).
Tezuka returns from a supposed meeting with Jun's gang and tells the team that they should cooperate with Jun for a better chance of survival. Ishida is skeptical and concerned about the matter, but Tezuka comforts him, saying "My predictions are never wrong". Tezuka asks everyone to give him their Advent Decks as a sign of trust. Initially reluctant, they all agree upon seeing that Jun's gang has also given Tezuka their decks earlier. The four then go to meet Jun's gang.
Elesewhere, as Ren is riding his motorbike, he is stopped by Asakura. Asakura claims that he remembers everything about his past as a Kamen Rider, telling Ren that they "used to be so close that they want to kill each other". Ren only responds by saying he knows Asakura's name. The two transforms and fight, with Ouja having the upper hand. As Ouja is cornering Knight, a Mirror Monster appears and prepares to sucker punch him. Luckily for Ouja, Kamen Rider Zolda appears and fend off the monster. Knight then goes and fights the monster, leaving Zolda and Ouja in the place. Ouja, appalled at the idea that Zolda protects him, charges at Zolda to fight. However, Zolda cancels his transformation, revealing himself to be Goro Yura. Asakura stops charging, surprised, realizing it is his archenemy Shuichi Kitaoka's servant. Goro calls Asakura his "master", and says that he is glad to see his "master" alive and well, and asks his "master" to allow him to return and work for him again. Asakura, uninterested, cancels his transformation and walks away, but Goro tries to hold him back. Asakura pushes him aside, saying he is not Goro's "master", commenting on how the Mirror World may have messed up the servant's mind. Howvever, Goro keeps holding on to Asakura, begging his "master" for him to come back. Asakura, frustrated, beats Goro up violently.
Outside, Ren is still fighting the previous monster, and managed to kill it with his Dark Visor. Suddenly, Kamen Rider Abyss shows up and attacks, saying that Ren should not be so careless after killing a monster. The two fight for a brief while before Knight finishes Abyss off with a Final Vent. Realizing he has killed a human, Ren is terrified. He lets out a scream of horror and anguish.
In an office, Asakura is seen with Goro, apparently has accepted him as his servant. Goro prepares some food for Asakura, who finds it delicious. Asakura wonders whether Kitaoka always eats these kind of good food or not. Pleased by the servant's cooking, Asakura decides to form a pact with Goro, stating that he will kill Goro the last.
In the real world, a young man is shown transforming into Another Ryuki with an Anotherwatch. Sougo and Geiz appear to stop it. A voice pleads "Tatsuya, stop...please...". The camera then looks into an hospital room occupied by a woman. It is implied the voice that pleads is hers.
In the Mirror World, Shinji's team finally meets Jun's gang, only to be betrayed as Tezuka has been on Jun's side all along. He tossed Jun's gang their Advent Decks, allowing to transform. Ishida is eaten alive by Raia's Contract Monster Evildiver, while Shinji and Kimura are helpless as they cannot transform. As the for Riders corner Shinji and Kimura, Ouja appears and uses his Final Vent on Tozuka, killing him. Ouja is apparently amused by the event, and sarcastically asks whether he can join before attempting to fight the Riders, with Zolda backing him up. As Ouja prepares to strike an untransformed Shinji, Knight appears and blocks the blow, saving Shinji.
Episode Tropes:
- Big Damn Heroes: At the end of the episode, Knight protects an untransformed Shinji from Ouja's attack.
- Face–Heel Turn: Miyuki turncoats his allies in favor of Jun's Riders, while Goro is siding with Takeshi of all people.
- Heroic BSoD: After killing Kamen Rider Abyss, Ren reacts in horror of what he caused.
- Laser-Guided Amnesia: None of the Riders have any memory of their past lives, except for their own names. Takeshi seems to be the only subversion as he still retained his own memories.
- The Oner: The opening shot after the opening shows Shinji's team against Jun's team, showing every fight while Ryuki fights against Gai, all in one moving camera shot while managing to keep weapon summons offscreen.
- Out of Focus: Both Zi-O and Geiz make a cameo fighting against Another Ryuki, all while transformed.
- Race Against the Clock: According to Sara, the new Rider War is to last a week at most, and by the beginning of the special, it's already on the 4th day.
- We Hardly Knew Ye: The new Abyss, Imperer, and Tiger all get killed by the end of the episode. Abyss and Tiger get hit the hardest as at least Imperer had a personality.
