It’s been eight years since Touma and his allies saved the world from Storious. With the help of Mei Sudo and his new friend Mamiya, Touma cares for a young 11-year-old boy named Riku, who lost his parents a while ago. Though Touma tries to be a father to Riku, the boy continues to go without smiling. Meanwhile, Kento has become a book translator and looks forward to his engagement with his girlfriend Yuina Tachibana.
However terrible events are unfolding within the members of the Sword of Logos as swordsmen after swordsmen disappear. Rintaro Shindo is dispatched to investigate this and encounters the new Kamen Rider Falchion, who claims to be Rintaro’s biological father. While Rintaro is confused by this revelation, the new Falchion attacks both Touma and Kento.
What is the secret behind the terrifying power of Amazing Siren, and how will Touma and the others resolve this one final loose end?