''Franchise/KamenRider Series/KamenRiderFourze & [[Series/KamenRiderOOO OOO]]: Movie War Mega Max'' is the third Movie War movie. Premiered in Japanese theatres on December 10, 2011.

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!! This movie contains examples of the following tropes:
* AllThereInTheManual: The names of Michal's Kamen Rider forms. The evil form that uses core medals is called Kamen Rider Poseidon, and the good form created from the water-powered belt is called Kamen Rider Aqua.
* BigBad: Lem Kannagi of [[Series/KamenRiderDouble Foundation X]].
* ContinuityNod: Tons given that this is a crossover (See below)
** In the [[ReCut Director's Cut]], Gentaro does his own take on [[KamenRiderStronger Kamen Rider Stronger's]] trademark introduction speech before his fight with the Dustards at Amanogawa High.
** It's implied that Foundation X backed the Kougami Foundation's research on Core Medals.
** Shotaro still uses the Lost Driver though it's unclear if it's [[spoiler:the one left to him by Skull or the one left to him by Philip after his "death"]]
** Both crossover cameos in ''Kamen Rider W Forever'' and ''Kamen Rider OOO Wonderful'' are referenced.
** Not to mention that Shotaro references Eiji's exclamation that "Riders look out for each other" from ''Kamen Rider W Forever''
** The Seven Legendary Riders are battling Foundation X in various countries. Back in the old days, when a Rider's predecessors would always appear in the final arc to help out, where did the guest star Riders say they were coming from? ''Fighting the evil organization's branches in other countries.'' This goes all the way back to the very first series, when Takeshi Hongo - his actor having recovered from the injury that necessitated Hayato Ichimonji's introduction - returned midseries. Fighting Shocker overseas is where he'd been. The only difference is this time we ''see'' the Seven Legendary Riders' overseas battles before they return to Japan to fight alongside the current heroes.
* {{Crossover}}: As the name of the movie suggests, it's a crossover between ''Series/KamenRiderFourze'' and ''Series/KamenRiderOOO''. However, [[Series/KamenRiderDouble Kamen Rider Double and Foundation X]] are featured as well, uniting the first three leading riders of the post-Decade Heisei era. In the wake of ''[[Film/OOODenOAllRidersLetsGoKamenRiders OOO, Den-O, All Riders: Let's Go Kamen Riders]]'', 2011 being the fortieth anniversary year of the series, the original seven Kamen Riders ([[Series/KamenRider Riders 1, 2]], [[Series/KamenRiderV3 V3, Riderman]], [[Series/KamenRiderX X]], [[Series/KamenRiderAmazon Amazon]], and [[Series/KamenRiderStronger Stronger]]) play a major role as well, where they are now known as the Seven Legendary Riders.
* EarlyBirdCameo: As each Movie War movie has featured the introduction of the upcoming [[SixthRanger Secondary Rider]] of the current show, Kamen Rider Meteor makes his first appearance at the end of the movie.
* [[spoiler:EleventhHourRanger: Kamen Rider Aqua returns from the future to help the Riders against Foundation X's forces, and give Eiji the Super [=TaToBa=] medals.]]
* ForHappiness: Shotaro is quick to voice how unimpressed he is with Gentaro - and just as quick to change his mind when he learns they have the exact same motive.
* HamToHamCombat: Shotaro and Gentaro, with Eiji and Philip looking on in (respectively) bafflement and amusement.
* MenDontCry: Gentaro claims that a man is only allowed to cry if he's lost his wallet or been dumped.
* {{Mooks}}: Through their research into the Gaia Memories, O Medals, and Cosmic Energy, as well as their partnerships with the Museum and Gamou, Foundation X boasts a vast foot soldier army of Masquerade Dopants, with a white suited variant transformed from Foundation X members using Gaia Memories, Waste Yummies, and Dustards. Poseidon also creates his own Waste Yummy army in OOO's portion by channeling the power of Uva's Core Medals, while in Fourze's portion Virgo leads a force of Dustards against Fourze and Nadeshiko as a favor from Gamou to Foundation X.
* LegionOfDoom: Foundation X uses the research its agents gathered to build an army consisting of various MonstersOfTheWeek as well as mooks from the three series ''Fourze'', ''OOO'' and ''W'', alongside monsters of the BigBad's own design, called "Mutamids". They're not actually the same monsters from their original series, but it's [[PropRecycling an excuse to reuse monster suits]].
* SealedGoodInACan: The Seven Legendary Riders are sealed inside Fourze and OOO's [[TransformationTrinket transformation trinkets]] by Kannagi. 1, 2, V3 and Riderman are trapped within Astro Switches while X, Amazon and Stronger trapped inside Core Medals. They are freed when Fourze and OOO perform a Limit Break and a Scanning Charge respectively.
* SuperMode: OOO's Super [=TaToBa=] Combo and Fourze's Rocket States.
* TrashTheSet: Being the finale to [=OOOs=], Cous Coussier is trashed following an attack by Yummies on Eiji and Ankh. Chiyoko tells them [[CrowningMomentOfAwesome that it will open just the same as it always has the next day.]]
* WhyDidItHaveToBeSnakes: Ironically, despite being a water-themed Rider, Michal is afraid of water. [[spoiler:He manages to overcome it in the end.]]
* TheWorfEffect: The Seven Legendary Riders are hit with this to show how dangerous Foundation X's threat is here in the movie.