A mysterious item called Pandoras Box has been found on an expedition to Mars. Once brought to earth, the box unleashes the “Sky Wall” that separates Japan into 3 parts and the government names these 3 islands Toto, Saito and Hokuto. In these walls emerges the monsters called SMASH. A young physicist, wearing a strange belt that houses 2 bottles, appears before them and he is called Kamen Rider Build!
Build will experiment various combinations, but only a few of them are perfectly compatible with one another. Called Best Match, the perfect compatibility includes Build’s base form: Rabbit Tank, nicknamed the Moonsault of Steel (Hagane no Moonsault). By combining various powers, Build can change the laws of victory.