Takumi Katsuragi was performing tests on human subjects using the Nebula Gas and it was going well, until his chief officer, Himuro, gets knowledge of what he has been doing and suspends his project. Soon after, Katsuragi gets scouted by a man who was interested on his work, and resumes his research for the Nebula Gas under the Namba Heavy Industries. How will this research affect the lives of those who were used as test subjects? Episode 1 takes place 3 years before the events of the Kamen Rider Build series.

The second episode of ROGUE, released on Kamen Rider Build Blu-ray Collection 2. Takes place between episodes 20 and 22 of the main Kamen Rider Build TV series.