Movie War Mega Max featured the first seven Kamen Riders (Kamen Rider 1, 2, V3, Riderman, X, Amazon, and Stronger) to commemorate the franchise's 40th anniversary. Trailers for the film shown during airings of Fourze also showed the return of Kamen Rider Double as well as a new form for Kamen Rider OOO. Unlike previous Movie War films, where they were divided into 3 segments, Movie War Mega Max is divided into 5 parts. Like previous Movie War films the secondary Kamen Rider of Kamen Rider Fourze, Kamen Rider Meteor, made his debut in the film.

All Might and Deku accept an invitation to go abroad to a floating and mobile manmade city, called 'I-Island', where they research quirks as well as hero supplemental items at the special 'I-Expo' convention that is currently being held on the island. During that time, suddenly, despite an iron wall of security surrounding the island, the system is breached by a villain, and the only ones able to stop him are the students of Class 1-A.

August 1995: Mikiya first meets Shiki in a white kimono during a snowing day. Later on, at the high school freshmen ceremony, Mikiya sees Shiki in the crowd and chases after her, introducing himself to her. But Shiki hides a big secret: A wave of bizarre murders occur around many loose ends, and no suspects. there are still many questions: Who to believe? Who is the murderer? What is the secret of Shiki? and especially: Who is SHIKI?

Tomoe Enjou is attacked by bullies from his old school and saved by Shiki Ryougi. He asks her to hide him at her place and admits that he killed someone. Several days later, there are still no broadcasts about the murder as if it didn't happen... and when the victims are found, they're alive and unharmed.