Not long after the events of Kamen Rider Gaim, Kouta and Mai, who are now ruling over a new planetoid as gods, are attacked by Megahex, a cybernetic being with ambitions of making the whole universe one with it's self in a symbiotic manner. Mai is taken prisoner by Megahex who then chases Kouta back to earth where he briefly fights Megahex to try and save Mai with the help of his old friends from Zawame City, but after struggling to hold their own against Megahex's army which is commanded by restricted versions of some of their old enemies. Meanwhile in another part of the Japan, Shinnosuke and the rest of the Special Investigation Unit are dealing with Phantom Thief Ultimate Lupin, a mysterious criminal who is running wild in the city. Moments after the case is solved, Megahex's invasion of earth expands to the rest of Japan and after a moment of confusion, Shinnosuke and Kouta team up to stop Megahex and save Mai along with the rest of the planet
