Ten thousand years ago, a battle royale known as the Battle Fight was waged by fifty-two immortal beings called the Undead, each representing a different species vying for dominion over the Earth. The victor was the Human Undead, securing humanity’s place as the planet’s ruling species, while the other Undead were sealed away in cards. In the present day, archaeological research accidentally breaks the seal, releasing the Undead to resume the Battle Fight. To counter this existential threat, the secret organization BOARD develops the Rider System, allowing specially chosen individuals to transform into Kamen Riders by using the power of the sealed Undead.

The story centers on four young men who are thrust into this conflict. Kazuma Kenzaki is a well-meaning but initially inexperienced man who becomes Kamen Rider Blade. His senior at BOARD, Sakuya Tachibana, transforms into Kamen Rider Garren, but he is haunted by fear and a mysterious illness stemming from his body's rejection of the Rider System, making him an unstable ally. They are soon joined by a third Rider, Hajime Aikawa, who operates independently as the mysterious Kamen Rider Chalice. Hajime is aloof and antisocial, living with a young girl named Amane and her aunt Haruka, who run a cafe. His true purpose and origins are a mystery, as he fights both the Undead and the other Riders. The fourth Rider, Mutsuki Kamijo, is a timid young man who desires power and becomes Kamen Rider Leangle, but he is immediately possessed by the malevolent Spider Undead that powers his belt, turning him into a dangerous pawn.

As the Riders battle the rampaging Undead, they uncover the truth behind the Battle Fight. The series is structured around key narrative arcs. The first arc sees the destruction of BOARD by the Undead, leaving the Riders without a central command and forcing them to operate on their own. A significant portion of the story focuses on the team attempting to free Mutsuki from the Spider Undead’s control. This arc explores themes of temptation and corruption, as the other Riders, particularly the guilt-ridden Tachibana, struggle to save their friend without destroying him.

The most critical revelation is that Hajime Aikawa is not a human, but the Joker, the 53rd Undead. The Joker’s purpose is not merely to fight, but to destroy the world if it wins the Battle Fight. As the last remaining Undead, Hajime is fated to battle the victor of the other 52 species, which would trigger the apocalypse. This creates an impossible situation for Kazuma Kenzaki, who has formed a deep bond with Hajime. As the series races toward its climax, the other Riders attempt to find a way to seal the Joker permanently, while Kazuma refuses to accept a future where his friend ceases to exist. In the final episodes, after the other Undead are sealed, Kenzaki is forced to fight Hajime directly. To save his friend without sealing him away, Kenzaki uses his King Form power to transform himself into a second, permanent Joker. By becoming an Undead himself, he creates an eternal tie with Hajime, ensuring that the Battle Fight can never conclude and the world is saved. The series ends with the two friends forever separated, Kenzaki wandering the earth alone as an immortal being to prevent Hajime from having to fight.

The narrative continues in the film Kamen Rider Blade: Missing Ace, which presents an alternate timeline where Kenzaki was forced to seal Hajime. Four years later, a new BOARD emerges with three new Riders, and the Undead are released once more. This leads to the resurrection of Hajime to combat the Albino Joker, a white version of his own kind. A novel sequel, set 300 years after the television series, follows Kenzaki’s centuries-long existence as the Joker and his quest to finally end his immortality.