"Alchemy" is a pseudo-science that attempts to create "gold" from various different combinations. One experiment to do so artificially brings to life a race of creatures, and the story revolves around an era where these experiments were being conducted in secret.
In the story, the artificial lifeforms known as the "Chemies", made to imitate things that exist in the world such as grasshoppers, were created from the art of the best alchemy techniques. Each and every one of them were carefully stored in "Ride Chemie Cards' until they were accidentally released all at once into the wild.
Our main protagonist, Houtarou Ichinose, is entrusted with the Gotcha Driver and the mission to recollect the Chemies that have been released into the world. Some are well suited for human companionship and if resonated with a good heart, Houtarou can fuse with them and transform into Kamen Rider Gotchard. However those that are influenced by human malice, there is a danger of them birthing monsters known as the Malgam. This is a story that explores the nature of good and evil, which are two sides of the same coin.
Will Gotchard be able to "collect them all", or will he let his carelessness mark the destruction of the world?
(Source: Kamen Rider Wiki Fandom)