Kamen Rider × Kamen Rider Fourze & OOO: Movie War Mega Max

Meteorites carrying a mysterious substance called SOLU fall to Earth. The original seven Kamen Riders – One, Two, V3, Riderman, X, Amazon, and Stronger – battle Foundation X's monsters around the world, but fail to stop the organization from retrieving the SOLU.

As one of the meteorites causes a time portal to open over Japan, Kamen Rider Poseidon emerges from 40 years in the future to fight the present day Kamen Riders. Kougami Foundation employee Erika Satonaka informs her friend Hina Izumi of the event while they head to the airport to pick up Eiji Hino. Hino rescues them from attacking monsters, but Poseidon attacks him before Satonaka's partners Akira Date and Shintaro Goto arrive to fight the mysterious Rider. Poseidon overpowers them as Hino reveals himself to be a resurrected Ankh while pulling several Core Medals out of Poseidon's body just as the real Hino arrives and weakens the enemy, though Poseidon escapes.

With Date and Goto hospitalized, Hino, Ankh, Izumi, and Satonaka regroup at the restaurant Cous Coussier, where Ankh refuses to explain how he was revived while Hino theorizes Poseidon might not be acting of his own free will. More monsters attack, forcing Hino and Ankh to fight them while Izumi and Chiyoko Shiraishi evacuate the restaurant. Poseidon returns and nearly kills Hino, but the person inside the armor stops the former and reveals he is Michal Minato, a heroic Kamen Rider from the future whose aquaphobia led to him being corrupted by several dozen Core Medals that emerged from a black hole and created Poseidon. Ankh retrieves more Core Medals from him before Poseidon separates himself from Minato in an attempt to travel to another time period, though he is destroyed by Hino and Minato. After Minato leaves, Izumi notices that Ankh disappeared. Hino reveals that the Ankh that fought with them also came from the future. Before they can leave, Lem Kannagi attacks them and steals Poseidon's personal Core Medals.

In the city of Fuuto, Shotaro Hidari stops a Foundation X convoy and destroys the monsters guarding it, though he causes the SOLU they were carrying to leak into a sewer. Meanwhile, Kannagi and his aides arrive at a secret airport to board his mobile spaceship lab, Exodus. The original seven Riders arrive to stop him, but Kannagi captures them.

During Amanogawa High School's cultural festival, the Kamen Rider Club puts on a presentation celebrating past Kamen Riders when a mysterious girl falls from the sky. Club member Gentaro Kisaragi saves her before joining the club in taking her to the hospital, where they learn her name is Nadeshiko Misaki before Dustards attempt to kidnap her. As Kisaragi transforms into Kamen Rider Fourze to stop them, Misaki transforms into Kamen Rider Nadeshiko. Soon after, the Virgo Zodiarts arrives demanding Misaki, but Kisaragi and Misaki drive her off. Kisaragi's friend Kengo Utahoshi asks where Misaki's Rider powers came from, but she runs off with Kisaragi, who takes her to the club's lunar headquarters, the Rabbit Hatch, where they grow closer. When Misaki takes off her space helmet to kiss him, Kisaragi realizes she is not human.

Utahoshi reveals that she is one of several Seeds Of Life from the Universe, or "SOLU", an alien lifeform that mimics what it sees and lacks a true consciousness, and college scientists have apparently come to study her. Kisaragi refuses to accept this and storms off. His childhood friend Yuki Jojima cheers him up, but he refuses to let Misaki go and takes her away from the scientists. Misaki is touched by his feelings, but the scientists reveal themselves as Foundation X agents and kidnap her. Kisaragi pursues them, but Kannagi's aide Katal impedes him. Misaki and Kisaragi's ally Shun Daimonji join the fight when Foundation X reinforcements arrive. The trio defeats them, but Kannagi captures Misaki and turns her into a SOLU Switch before defeating Kisaragi and leaving him at Katal's mercy. Kisaragi uses an Astroswitch that Misaki created to destroy Katal. Soon after, Hino arrives to ask for Kisaragi's help in defeating Foundation X.

Meeting up with Hidari and his partner Philip, Kisaragi and Hino learn that Kannagi betrayed Foundation X to achieve world domination. As Kannagi's forces confront the Riders, Hidari and Philip stay behind to hold them off and repay Hino for helping them during NEVER's attack on Fuuto[lower-alpha 1] while Hino and Kisaragi pursue Kannagi, finding and freeing the captive seven Riders on the way. As the seven Riders offer to stay behind and hold off Kannagi's forces, Minato returns to give Hino three Core Medals from his time to assist in the mission. Hino and Kisaragi reach the Exodus, where Kannagi uses Poseidon's Core Medals, the SOLU Switch, and the Gingaoh Driver to transform into Super Gingaoh and overpower the Riders. Hino and Kisaragi use the future Core Medals and Misaki's Astroswitch respectively, to fight Kannagi on equal footing before destroying him and the Exodus. Becoming an energy being, Misaki returns to space while the Riders return to Earth. As Hino resumes his journey to resurrect Ankh, Kisaragi encounters the real Misaki on his way back to school.

In a post-credits scene, Kamen Rider Meteor defeats Kannagi's forces' remnants before wondering whether he will find his target at Amanogawa High School.