ZAIA Enterprise Japan has disappeared and Gai Amatsu (Kamen Rider Thouser) is planning to start a new company (Thouser Intellion) with the help of Secretary type Humagear Rin. He had decided to Innovate. But something happens to him. He is in pain. That's when Kuroto Dan (Kamen Rider Genm) separates himself from Gai Amatsu's body.
Because of Ark, the Ultimate Bugster Virus who was in the Internet had revived and infected Gai Amatsu. Gai Amatsu transforms into Kamen Rider Thouser and tries to get rid of the virus. But for some reason his suit is black (Kamen Rider Zaia).
Since the story takes place after the fall of ZAIA Enterprise JAPAN, timeline could be directly after Zero-One Others: Kamen Rider MetsubouJinrai or after Zero-One Others: Kamen Rider Vulcan & Valkyrie.
Even though Kamen Rider Zaia's suit is used, but the form will be still dubbed as 'Kamen Rider Thouser'. Guess we have tell the difference based on the Progrise Key in the Thousandriver.
The spin-off will be directed by Satoshi Morota and that's the reason why you see the river scene being used. Yuya Takahashi who is known for his works of both Kamen Rider Ex-Aid & Kamen Rider Zero-One will be in charge of the screenplay for this spin-off.