Unidentified creatures called Smash have appeared at Touto, Sento Kiryu turns into Kamen Rider Build to fight against them. One day, he saves a guy named Ryuga Banjo who escaped from a prison.

With no time to mourn for Kazumi's death, Sento, Ryuga and Gentoku brace themselves for the final battle against a full-powered Evolto.

After the sacrifice of his companions, it falls to Sento to protect the world by defeating Evolto once and for all.