Desperate and terrified, Kotaro Minami escaped the clutches of the ancient secret society, Gorgom—but in doing so, left behind his best friend and adoptive brother, Nobuhiko Akizuki. Both have been transformed into beings of immense power in order to serve Gorgom's dark purposes. If left unchecked, Gorgom will destroy humanity and eliminate all traces of its civilization and culture.

Now the powerful cyborg known as Kamen Rider Black, Kotaro will use the powers Gorgom gave him to preserve justice and freedom, as well as to save his brother. But Gorgom has their own plans for Nobuhiko—and if Kotaro can't save him, he may have to fight him!