Emu, a clumsy pediatrics intern takes young Sota, who is suffering a mysterious illness, to the announcement event of the latest "Mighty Action X", Sota's favorite game. Emu became a medical doctor in order to save someone's life because his life had been saved once through surgery.

However, a mysterious virus emerges from Sota's body and the event turns into chaos. Emu hears from Asuna, an employee of the Ministry of Health, that transforming into a Kamen Rider can save the patient. Emu takes on this "operation" and inserts the Mighty Action X Gashat into the Gamer Driver, transforming into Kamen Rider Ex-Aid!

With Christmas around the corner, Emu runs against time to defeat the Salty Bugster again and save another patient, while Kiriya's investigation leads him closer to the truth, but his life is in danger when he faces Kuroto's new power.

In order to deprive Ex-Aid of his transformation abilities, Cronus decides to target Parado directly. Para-DX without Emu is still not perfect, so he is beaten easily. Even so, Ex-Aid cannot stop chasing Cronus. However, Cronus was planning an underhanded scheme. Nico's game disease, which just started to show symptoms, belongs to that of Gamedeus, the Last Boss of Kamen Rider Chronicle! In order to defeat Gamedeus, Ex-Aid has to defeat both Graphite and Parado! Just then…Parado's hands begin to shake violently…