The final chapter of Kamen Rider Build is set in the new world created by Sento Kiryu where the memories of the old world are returned only to those who were subject to human experimentation by Faust. In the story, Deputy Officer of the United Nations Alliance, Simon Marcus(Michael Tomioka), who aims to conquer the world with the terrorist organization Downfall along with the mad scientist Keiji Uraga, attack the Riders with overwhelming strength. It is only on Kamen Rider Grease that the enemy's transformation release abilities did not work. In order to defeat this new enemy and rescue Misora, Kazumi Sawatari must create a new item ... but it is told that it requires the life of the Three Crows. The ultimate decision must be made.