After Gremlin's defeat, Haruto is lost in the World of Kaijin, a parallel world where it is inhabited by monsters created by an entity called Amadum who wants to steal the powers of the Kamen Riders for himself. Using the Legend Rider Rings, he will summon 13 Heisei Kamen Riders.

Haruto and Kosuke must defeat Amadum before he can use the Rider Rings to transform their world into a kaijin's paradise.