In a world where the dreams people have at night have the power to shape reality, a clandestine organization known as CODE deploys special agents to combat a rising threat. These agents are tasked with entering the dreamscape to neutralize Nightmares, monstrous entities born from subconscious fears and desires that can bleed into the waking world and cause catastrophe. The organization’s most effective weapon in this hidden war is the Zeztz Driver, a belt worn across the chest that allows an agent to transform into Kamen Rider Zeztz.

The central figure of this conflict is a young man named Baku. As an agent of CODE, he is designated Code Number: Seven and fights as Kamen Rider Zeztz. His missions take him on a surreal journey through the dreams of various individuals, each one a unique and often bizarre scenario. He must protect the dreamers, confront the Nightmares that haunt them, and prevent these psychic disturbances from causing real-world disasters, such as biological weapons being hidden in a restaurant or a giant meteorite approaching Earth. Within these dreams, Baku is often not himself but takes on roles fitting the dream's narrative, such as a high school student, an employee at a space center, or a villager in an RPG-like adventure.

Baku does not work alone. He is supported by his fellow agents and friends, including Fujimi, Nasuka, and Minami, another agent who becomes a key figure in his life. Their home base, the CODE headquarters, is run by enigmatic superiors like Zero and Three. However, the organization has a ruthless side, and Baku begins to question his role as an agent when he learns of a traitor within their ranks. That traitor is Nox, a former CODE agent who now operates as a renegade Rider, possessing his own transformation belt. Nox is a cold-blooded and powerful enemy who seems to have a personal history with Baku, one that involves erased memories and a past encounter when Baku was a child attending a cram school where Nox was the instructor.

As Baku delves deeper into his missions, he uncovers a larger conspiracy. He learns of Nem, a mysterious girl whose subconscious may be the very source from which all Nightmares are born. His mission parameters shift from simple extermination to the rescue of Nem, who has been abducted. This puts him into direct and repeated conflict with Nox, who has his own plans for the girl. The battles between the two Riders escalate, forcing Baku to confront the truth about his own past, the true nature of CODE, and the reason his memories have been tampered with. In a desperate struggle, Baku unlocks new forms, such as Zeztz Catastrom, to match his enemy’s power.

Ultimately, the conflict becomes deeply personal. The line between the dream world and reality blurs as Baku discovers that his own experiences may have been manipulated, with his relationships and sense of family questioned. The series follows his journey as he navigates the harsh directives of CODE, his rivalry with Nox, and his quest to save Nem. It builds towards a final confrontation where Baku must shatter the endless nightmare and decide the fate of both worlds, culminating in the emergence of a new series successor, Kamen Rider MY-TH, which will continue the legacy with a theme centered on myths, tales, and urban legends.