An olympic swimmer disappears suddenly right before the eyes of his teammates, and a young man swimming in the ocean gets kidnapped by a monster who appears.

The Rider Scouts are at training camp on Hatsushima Island, where mutant Speargun-Sea Lion has established an undersea outpost to manufacture poison gas. Destron plans to transport the chemical weapon via an underground canal to Mt. Fuji, where the winds will spread the gas over hundreds of miles. Kazami Shiro orders the scouts to evacuate, but they refuse when they realize one of them is missing. Can V3 stop Destron in time?