Kamen Rider Genms -The Presidents- is a two-episode TTFC-exclusive crossover miniseries between Kamen Rider Ex-Aid and Kamen Rider Zero-One. In the miniseries, Gai Amatsu is infected with the Bugster Virus by a revived Kuroto Dan, who wishes to bring back Genm Corp. for his own malicious purposes.—Kamen Rider Wiki

With the collapse of ZAIA Enterprise Japan, Gai Amatsu establishes a new company with a secretary-type Humagear Rin. But something went wrong with his body, and Kuroto Dan appeared and separated from his body. The Bugster Virus, which was wandering in the internet and was revived by Ark and infected Gai. Gai transforms into Kamen Rider Thouser to try and get rid of the virus, but Thouser looks black for some reason.—Kamen Rider Wiki

It's been sometime since Kamen Rider MetsubouJinrai blew up ZAIA Japan's headquarters, and Gai's looking to rebuild. Meanwhile, we learn that the Ark found another potential wielder before it and As were blown to smithereens, one who is also a source of ambition and hatred that transcended humanity like itself. Gai Amatsu also has a new friend, a secretary HumaGear he named Rin, to help him build a new company. Unfortunately, one outbreak of Game Disease later the accumulated stress and the choice to set up office in MetsubouJinrai.Net's former hideout prove to be even worse for his health than expected. Things go downhill and into the rabbit hole from there. First, the Bugster that spawned from him is the Game Master, viral god, KUROTO DAN HIMSELF. Second, Kuroto proceeds to steal said secretary and try to convince her to join him instead. Now Gai has to fight him both for Rin's sake and his own life, because otherwise he will disappear and give the viral lunatic another lease on existence.—TVTropes