One day, Flash wakes up out of the blue to find himself in possession of an unusual device and no memory of how he got it. And when his home is invaded by shape-shifting monsters, he finds he will need to use that device to save his friends and family.

With the powers he now possesses, Flash must take on the mantle of hero. Along the way, he will face many challenges and uncover many secrets. He may end up sacrificing everything in order to protect his world, but he will grow stronger through those sacrifices. And eventually, he may learn to live up to the title given to him. The title of a Kamen Rider.

18 years ago, a strange, celestial body crashed just on the outskirts of Canterlot. A mysterious box was found and studied in the Canterlot Research Facility. Despite the time and effort, its secrets have yet to be unearthed....until now.

6 months had passed since the disappearance of Micro Chips' brother, Gizmo. But upon a mysterious delivery, he and his friends Flash and Sandalwood work to crack the mystery surrounding the incident.

Now, rampaging monsters run amok in Canterlot City, emitting a strange gas that turns others into the same creatures. What's worse, Equestrian Magic seems to have no effect. Fortunately, Gizmo had left something behind. A particular system needed to combat these creatures and uncover the truth.

Flash Sentry now takes up the mantle, to build the winning formula.

In collaboration with Banshee531

Kamen Rider Build Crossover.

Edit 6/24/2020: This story had been featured. I am surprised and grateful. I didn't expect it to happen.