After a brief fight with it in its samurai form, Kotaro transformed into Kamen Rider BLACK. Cobra Mutant escaped into the timestream to pursue two samurais. Cobra Mutant caused them to use the machine to alter people and stop time. Kotaro followed them into the timestream and ended up on the face of a giant clock where Mamoru was being held. Cobra Mutant attacked Kotaro who then ended up hanging from the side of the clock. The two samurais arrived and ended up helping Mamoru.
When Cobra Mutant started to attack them, Kotaro transformed into Kamen Rider BLACK who kept Cobra Mutant occupied. When the samurais freed Mamoru, Kamen Rider BLACK used the broken big hand of the clock to chop off Cobra Mutant's tail and stab it.
Once Kamen Rider BLACK killed Cobra Mutant with the Rider Kick, the device was destroyed and time was set back to the way it was.