Monsters are roaming in the Mirror World. Those monsters will hunt the humans in the real world as their food, but they cannot stay long in the real world. These monsters have different interest upon their prey. 

Once choosing his target, the monsters will keep following their prey on the nearby mirror and will wait for the right time to pull the target out from the real world and consume the target in the Mirror World. However, targeted human which possess the Seal Card can prevent the monster from getting near and keep them away. 

All over the city, innocent people are disappeared, amongst this chaos in the real world, some Kamen Riders are born. They are the monster slayers and can even travel into the Mirror World in a flash using a machine called Rider Shooter. Although they can only stay in the Mirror World for a short period of time.

Beside the Rider Shooter, a Kamen Rider also posses several devices. Most essential is the Vent Deck. The other one is a Visor, which will appear after the user is transformed into the Rider form. 

The Vent Deck is a card deck containing a full set of basic cards. These basic cards can be used to summon some equipments, seal monster away, or even made a contract with the attracted monsters. While the Visor, is the device where the Riders can insert their cards. Note that the Visor location between each Kamen Riders may be different.

A basic Kamen Rider are weak, they need to be stronger if they want to survive in the Mirror World. A Rider must use their Blank Contract Card to made a contract with one of the thirteen strongest monster.

Once contracted, the monster will become the Rider's Advent. Their old basic cards inside the Vent Deck will be replaced with the new set of Advent cards based on the ability of the contracted monster. Their Vent Deck will show a corresponding crest based on the shape of the contracted monster.

Their Rider form appearances and motif will also changes according to their Advent. They are now far more powerful and can summon their Advent to aid them in a battle. They also can stay longer in the Mirror World.

In order to change to the Rider form, the users must show his or her Vent Deck against any mirror-type object (including water), then a V-Buckle (a belt) will appear on the mirror and wrapped on their waist.

Shouting the word "Henshin!" followed by inserting the Vent Deck into the V-Buckle will transform the user into a Kamen Rider form.

Having an Advent is not without risk at all, the Riders must keep their Advent steady and never let them hungry, or the Riders themselves will be consumed by their own Advent. 

To feed the Advents, the Riders must kill a detected nearby monster in the Mirror World and swallow the life force of the creatures they destroy. In exchange for feeding them, the Advent as well as the Rider will become more and more stronger.

It was then revealed later that the cause behind these monsters and Riders existence are the work from a man called Shirou Kanzaki, a genius scientist. Mysteriously disappeared in Seimein University after the accident from an unknown experiment.

His intention is to make all of the 12 Kamen Riders fight each other until there is only one left. The last survivor must fight a battle against Kamen Rider Odin.

The creator of the Advent Cards has only one rule: that there can be only one Kamen Rider. The others must be eliminated.

The life energy of the dead Kamen Riders will be gathered together and used to grant the sole victor a single wish whatever they want. The battle between thirteen Advents has begun.