Mark Okita

The story revolves around the nine parallel universes of the previous Heisei Kamen Riders merging into one, something that would destroy all the worlds. To prevent this, Tsukasa Kadoya transforms into Kamen Rider Decade and is told that to save his own world he must travel with Natsumi and her grandfather to the other Kamen Riders' Worlds called A.R. Worlds (Another Rider's World or Alternate Reality World) and defeat the other Kamen Riders. (wikipedia)

Houtarou is summoned to a parallel world by Houou Kaguya Quartz to protect it from the evil Hundred. Kaguya seeks power from past Kamen Riders and wants to make Houtarou "gorgeous" to gain his help.