Celebrating ten continuous years of Kamen Riders in the Heisei Era of the franchise (1999-2009). Ten separate Kamen Rider Worlds are mysteriously merging together, which will eventually destroy them all! To prevent this, and restore the security of these Worlds, the mysterious Kamen Rider Decade must travel to each and eliminate the anomalies while trying to figure out what caused these mergings to begin with! But... does that also include him destroying all Kamen Riders to accomplish this?

In 1986, the Shima Foundation secretly funds individuals to seek out and destroy stained-glass monsters called Fangire which feed on humans’ Life Force to survive. Though the Fangire Hunters are skilled in hand-to-hand combat, they are usually no match for the cleaver Fangire who can also disguise themselves as humans. That is, until Kamen Rider Kiva appeared… Over twenty years later, in 2008, the Fangire are still a threat, and the next generation of Hunters is tracking them down. A strange young man, Wataru Kurenai, is allergic to, and fears the world around him. And he lives to one day create the ultimate violin like his arrogant womanizing deceased father did. But Wataru must fulfill his destiny as the new Kamen Rider Kiva to protect humanity from the Fangire. But the Shima Foundation fears what the appearance of Kiva could mean, since he almost destroyed the world the last time.

Ventara is an alternate dimension that exists behind all mirrors. For everything that exists on Earth, there is a copy of it on Ventara as well, and more. The alien General Xaviax invaded Ventara and began a war to conquer it. To defend themselves, the Ventarans created twelve armored superheroes, the Kamen Riders, who use individual Advent Deck cards to command powerful Contract Beasts for attack and defense. Xaviax defeated all but one of the Kamen Riders and stole their Advent Decks, eventually conquering Ventara as well. Unfortunately, because the Ventaran Riders had all been defeated & Vented (banished forever into the Advent Void), Xaviax had to find the genetic equivalents on Earth since the Advent Decks were DNA-locked. Finding specific humans on Earth, Xaviax disguises himself as a different person each time, and tricks them into serving him. One Advent Deck, however, fell into the hands of a young man on Earth named Kit Taylor. Working together with Len, the mysterious original Kamen Rider Wing Knight, Kit reluctantly uses his own Advent Deck to become Kamen Rider Dragon Knight in order to find his disappeared father, and stop General Xaviax and the ten other corrupted & selfish Kamen Riders from conquering Earth.