The story begins with a regular day with Takumi, Mari and Keitaro, which suddenly turns into a musical number thanks to a mysterious new boom box created by Smart Brain. As it turns out, Yuji, Yuka and Naoya have the same boom box and are greeted by a rapping Smart Lady who reveals that it's part of a new plan by Smart Brain to invoke people into singing all over Japan. The reason: To get the three Rider Belts back!