Season 1

Takumi Katsuragi conducts experiments on human subjects using the Nebula Gas, with promising initial results, until his chief officer Himuro uncovers the operation and suspends the project. Soon afterward, Katsuragi is approached by a man intrigued by his work and resumes Nebula Gas research under Namba Heavy Industries. How will this research affect the lives of the test subjects? Episode 1 takes place three years before the events of Kamen Rider Build.