Shortly after Shinnosuke Tomari (Ryoma Takeuchi), Kamen Rider Drive, eradicates the Earth of the 108 Roidmudes, he is put on a special assignment to investigate paranormal activity elsewhere in the city. He heads to the temple Daitenkū-ji to speak with witnesses Akari Tsukimura (Hikaru Ohsawa) and Onari (Takayuki Yanagi), but as he is interrogating them, a Gamma appears and steals an Eyecon from the temple, prompting Takeru Tenkūji (Shun Nishime), Kamen Rider Ghost, to fight it, even though to Shinnosuke it seems that Kamen Rider Ghost is fighting no one at all as he cannot see the Gamma

Leading an expedition in Germany, Kougami unseals the resting place of one of the alchemists that created the Core Medals in an effort to retrieve a set of lost Medals. However, upon the seal being undone, millions of Cell Medals erupt into a tower while creating a magical barrier that causes parts of Japan to flip over. As the alchemist reveals her plans to become a new OOO, Eiji and his friends are thrown into the past and must find their way back home and stop the alchemist before she destroys the world!

When a powerful new Makamou attacks and defeats Hibiki, Asumu dives into Takeshi’s history and discovers a book that details the ancient Oni and a boy sharing his name. As he reads on, he learns of the tensions between humans and Oni as they struggle to stop the Orochi Makamou, but will he discover the key to defeating the new Orochi in the present?

Confronting a mysterious enemy, the Pteranodon Yummy, Kamen Rider W receives aid from Ryu, who has infuriated Akiko by being too busy to attend her wedding! Akiko demands to know why he even became a Kamen Rider in the first place and the Pteranodon Yummy as it uses the Memory Memory on her to give her the answer to her question. Meanwhile, at the Kougami Foundation, Kousei Kougami talks of the most avaricious man in Japan, Oda Nobunaga, whose final resting place was found. With Doctor Maki overseeing the process, Kougami commissions the creation of a Cell Medal homunculus based on Nobunaga himself. Later, Kamen Rider Core begins to destroy the area as Kamen Riders OOO and W team up to fight the enemy. However, Kamen Rider Core proves too powerful and the two go underground to destroy the Gaia Memory’s source of power to even the odds.

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.