(Author's note: the story takes place directly after the World of Amazon rather than taking place at an undefined point during the later half of Kamen Rider Decade and also knocks Episodes 30, 31 and Movie Wars 2010 out from continuity. Bear this in mind as any facts/developments raised in those episodes/movie can/will be contradicted.)
In the corner of a stadium stood a man wearing pink and black armour with what looked like green eyes. Another wearing black and green armour with what appeared to be red eyes. Suddenly, they charged at each other.
To find out how these two ended up together, we must first go back about a week.
Our story begins with Tsukasa and his friends, Natsumi, Yusuke, Kivaara and Natsumi's grandfather and his on-and-off rival Daiki. They were leaving the World of Amazon as grandpa pulled down on the chain to pull down the next mural of a house. Usually Tsukasa knew whichever world they had landed in, but this time he did not.
"That's it? This world is the World of Someone's House? Well, screw you all," said Daiki as he went off to find treasure to steal. Tsukasa meanwhile went off to take photos of the new world.
The following day Tsukasa developed the photos, but something unusual happened. Usually, Tsukasa's photos came out semi-clear, but this time they were coming out perfectly clear. "This has to be it," said Tsukasa when he showed the photos to everyone, "this has to be my world." He then turned to look at the mural, "And that house has something to do with it."
"Everyone, come and look at this," said Kivaara. The others then followed her into one of the hallways of the photo studio. "Look, see, this photo has a house that looks like the one in the mural," she said. At that moment, a memory stirred within Tsukasa's mind of a man leaving the house. He then reached inside his pocket and took out a small key. "We need to get to that house," said Tsukasa to Natsumi and Yusuke.
"But do you even know they way there?" asked Natsumi.
"I have a feeling."
About an hour later (following Tsukasa's gut instinct) they arrived at the house on their motorcycles (Natsumi riding on Tsukasa's). Tsukasa walked toward the door with the key in hand, placed it into the lock, turned it and opened the door. Inside they could hear someone playing the piano. Following the source of the sound, the group ascended a flight of stairs and found a young girl of about 20. Hearing footsteps approaching she stopped playing, turned, got up and suddenly hugged Tsukasa. "You came back!"
Tsukasa didn't know how to react. Here was this girl, presumably his sister, or cousin, maybe even fiancée glomping him and he didn't even have any memory of her. "W-Who are you?" he asked the girl.
The girl stopped embracing him. "You don't know who I am?" she said, sounding forlorn. Tsukasa shook his head. "I'm Sayo, your sister, remember?" But Tsukasa still had no idea.
"I see the prodigal son has returned," said a male voice. The group turned and a middle aged man stood before them.
"Who are you?" asked Natsumi.
"I am Nobuhiko Tsukikage," said the man, "I am Sayo's butler."
"I'm afraid I don't have any memory of any of you," said Tsukasa.
"Then in that case, it is good that you managed to find your way back. Have you tried looking around, perhaps something may jog your memory."
"Here, I'll walk with you," said Sayo, taking him by the hand.
"This is Tsukasa's old house?" said Yusuke to Natsumi, "he sure lives fancy."
So Sayo took Tsukasa for a tour around its numerous rooms.
"This is the dining room," said Sayo, showing it to Tsukasa.
"I'm not remembering anything," said Tsukasa.
"Not even the sea cucumber incident?"
"No, what happened."
"Well, Nobuhiko cooked sea cucumber that day, but when you tasted it, you thought it was so disgusting you brought it back up again and swore never to touch it again.
"I guess that's why everyone says I have something against sea cucumber."
Sayo even showed him the garden, but nothing else was triggered in Tsukasa's mind. "See, this is where you used to play soccer all day," said Sayo. Tsukasa just stared at the green field. Just then, Nobuhiko emerged from the house carrying a folded paper with him. "Something has just arrived for you master Tsukasa," he said.
Tsukasa took the paper off him and opened it. It was an invitation to a Kamen Rider tournament. He then took the paper inside to show it to the others.
"A Kamen Rider tournament?" said Yusuke, "what does that mean?"
"According to this," explained Tsukasa, "it means that every Kamen Rider has to battle against each other in order to get the top prize."
"Which is?"
"It doesn't say."
A week later when Yusuke and Tsukasa turned up at the stadium where the event was being held, where dozens of Kamen Riders had gathered, some they recognised from their travels, others they had never seen before. Natsumi and Nobuhiko were attending the tournament as well and were seated in the spectator's area. Just then the master of ceremonies began his speech. "Kamen Riders of all worlds, welcome to this tournament to decide who among you is the strongest, the prize for which will be revealed to you when you win," he said, "now then, let the contest begin."
Seeing the emcee, Tsukasa suddenly felt a memory come to the surface of his mind. He saw the emcee on a beach. His face was turned away, but now was turning toward him. Behind him was the sun, and before he could see anymore he snapped back into reality and found Yusuke was tapping him on the shoulder.
"You OK?" he asked.
"The emcee," said Tsukasa, "I think I know him from somewhere, but I can't figure out where.
And soon enough Tsukasa, having transformed into Decade, found himself fighting against Kamen Rider Black RX.