From the introduction:
The year is 2004, and the world is much like the one you originally came from. However, the fate of mankind hangs in the balance as an ancient battle royale has begun once again in the present day. Ten thousand years ago, this battle raged between deathless creatures called the Undead in the name of supremacy over the planet. A singular Human Undead won the royale, and the rest were sealed away into cards while humans became the dominant species on the planet. Now, these Undead have been unsealed and the battle begins anew, with the fate of both humanity and the future of the planet at stake. To prevent this, a secret organization called BOARD has created the Rouze Card System to create two Kamen Riders by partially merging them with Category Ace Undead, giving them the power to fight and seal the Undead. Thus, Kamen Rider Blade and Kamen Rider Garren were born.