"Sabimaru-Senpai is gone."

Houtarou laments his immaturity and weaknesses in the face of fleeting, yet endless suffering. How can he help Sabimaru? Just then, Houtarou is invited by Kajiki to call out a UFO whose sighting has recently become more frequent.

Houtarou and his friends find a possibility to rescue Sabimaru by befriending UFO-X. However, the Three Dark Sisters and Sabimaru with the Dreadriver appear before them! Equip yourself with the Number 10 Chemy and Change to Super Gotchard now, Houtarou Ichinose!

The Dark King Gigist appeared at the Union headquarters. Supana, angered by Gigist's defeat of one of the Union leaders, transforms into Kamen Rider Valvarad. He boldly attacks Gigist, but...

Gigist appeared in front of Houtaro, took away Skebows' card, and turned it into a Malgam. Houtaro transforms into Gotchard and challenges the Malgam to save Skebows, but he feels something strange in the battle...!?

It was the Underworld King Germain and Gaelya who descended to earth from the Ouroboros world. Their target is Fuga, the father of Rinne, the most powerful alchemist. The Three Dark Kings, including Gigist, are closing in on Fuga.

Houtaro, Rinne, and Supana rush to the scene and transform into Riders! They confront the Dark Kings to protect Fuga, but they are cornered by the unprecedentedly vicious power used by Germain and the others...!