|This article is about a/an alternate reality/A.R. World (another rider's world) in Kamen Rider Decade.|
The World of Negatives (ネガの世界 Nega no Sekai) is a parallel world created in the image of the one Natsumi comes from, but is actually home to monsters who assume human form and serve Dark Riders (ダークライダー Dāku Raidā) who hunt down any real humans that are still alive. The backdrop used to enter this world depicts the destruction of the first world caused by the monsters' conflict among themselves, the scene itself matching up with the mother and child running from the flames in the first episode, just before the event was frozen by Wataru Kurenai. In this A.R. World, Tsukasa wins a contest and becomes a male model. Daiki's treasure in this world is the K-Touch.
With their tour of the nine Kamen Rider Worlds complete, the Hikari Studio group return to their home world, finding it as it was before the monsters appeared. Tsukasa is the only one who feels something is not right and questions the reason for the journey. Before Yusuke or Natsumi can attempt to answer, they encounter the violinist Otoya Kurenai, who thanks them for saving the world and tells Tsukasa that he earned a world of his own for his journey. As Natsumi goes out to see her friends from a club named the TG Club and their teacher for a high school reunion, Tsukasa and Yusuke have a short encounter with Narutaki who congratulates him on getting his own world. Later, Tsukasa becomes the ten thousandth (10,000th) customer in a restaurant and wins a free meal and a vast amount of money. Tsukasa enjoys the upper class lifestyle until he takes his leave during an arranged marriage meeting. Yusuke is forced to take his place, but he runs off upon realizing that the women are actually Raydragoons in disguise. Elsewhere, while hanging out with her TG Club friends, Natsumi tries to figure out where her friend Chinatsu is, with the TG Club members take the opportunity to run off, and the teacher attacks a couple as Alternative. Tsukasa runs into Otoya, who mentions a treasure before transforming into Dark Kiva and challenging him, along with the other members of the TG Club who become Ryuga, Dark Kabuto, and Orga. With Diend arriving to back him up for this treasure, Decade engages Dark Kiva in battle. Before Decade can use his Final AttackRide against Dark Kiva, the attack is interrupted as he falls and reverts back to human form. As the Dark Riders leave, Tsukasa sees that all of his Rider Cards had gone blank. Elsewhere, Natsumi is still searching for Chinatsu when she passes by someone in dirty and ragged clothing who turns out to be another Natsumi.
After Tsukasa suddenly loses his powers, he learns from Kaito that he is actually in the World of Negatives. He is later seen by a talent agent and receives an offer to be signed onto his agency to become an idol. Although Yusuke acts as the voice of reason, he gets caught up in the fame seeking as well. Meanwhile, after her school friends return while waiting for Chinatsu, Natsumi suggests that they dig up the "treasure" they buried together, only to find that a parking lot was built over it and Chinatsu moved the treasure to another place before this. Once Tsukasa finds Natsumi, he attempts to tell her that they are not in the real world, but Natsumi refuses to accept this as she runs off to the TG Club's hideout where she also meets the World of Negatives' Natsumi. Confirming that Tsukasa was right and revealing that the world was a haven for monsters with only a few human survivors left in hiding. Furthermore, the Dark Riders had murdered the real TG Club members' Negative counterparts and assumed the males' identities with the other Natsumi the only survivor. It was then that the Dark Riders arrive to further prove the other Natsumi's words as they force her to dig up the treasure that she buried: the K-Touch. Otoya was prepared to give it to Tsukasa so he could become the ultimate Rider until Diend intervened by attacking the Dark Riders. Though Otoya offers him the world on a silver platter, Tsukasa refuses and regains his powers. Using the K-Touch, Tsukasa transforms into Decade's Complete Form, destroying Ryuga, Orga and Dark Kabuto with the powers of Ryuki Survive, Faiz Blaster, and Hyper Kabuto, respectively, as Otoya escapes destruction. After the other Natsumi bids farewell to her counterpart and Tsukasa, to whom she gave the K-Touch, the group returns to the Hikari Studio to resume their journey home.
- Otoya Kurenai
- Natsumi Hikari
- Hirohiko Sato
- Kenji Sakata
- Kazuyoshi Aoyagi
- It can be implied that this world is affiliated with a certain organization because of the existence of multiple Rider belts from different shows.
- But most importantly, the K-Touch has existed in this world for possibly many years, and it is a power-up device exclusively made for Decade.
- The post-apocalypse-style world might be a reference to the Kabuto movie God Speed Love and the 555 movie Paradise Lost.
- It can be implied that Smart Brain and ZECT existed in this world because of the existence of two of their Riders.
- When Decade was defeated, it is unknown if the Riders of this world were restored as well.