Waniida from Black Satan attacks a young boy and long-time Kamen Rider ally Tōbei Tachibana, but Kamen Rider Stronger and Electro Wave Human Tackle run in to make the save. However, Shigeru and Yuriko fall into separate traps and are captured by Black Satan. Yuriko fights for her life as Shigeru is brainwashed by the organization.

When military experiments go haywire and trigger an atomic bomb, the consequences are of epic proportions. A monster arrives in the midst of the nuclear fallout, and Japan's defenses are helpless against it. Mankind's only savior is an irradiated water goblin from Japanese folklore called the "Death Kappa." The two rival monsters must go head-to-head in the ultimate battle between good and evil!