An IntercontinuityCrossover ([[WesternAnimation/CodenameKidsNextDoor the second]] [[TheGrimAdventuresOfBillyAndMandy one on]] CartoonNetwork) between the two Creator/ManOfActionStudios series ''WesternAnimation/Ben10UltimateAlien'' and ''WesternAnimation/GeneratorRex'' that aired November 25, 2011.

A mysterious rift begins to open in the skies of Manhattan. Meanwhile, Rex is sparring against [[TheStoic Agent Six]] While musing about his need for a "Theme song," they are interrupted and sent to close the rift. Providence appears on the scene to stop the rift, but are unable to do anything before Rex arrives with Six and [[DeadpanSnarker Bobo]]. [[HotScientist Doctor Holiday]], then gets a scan that shows something is coming through the rift, and Rex finds himself (unknowingly) fighting against, [[{{WesternAnimation/Ben10}} Ben Tennyson]], in the form as Humongousaur. During their skirmish, a strange metallic creature also appears and heads into the city, with Bobo and Six on it's trail. Ben is finally defeated by Rex, when trying to protect bystanders, revealing his true human form. Then, Six appears with the creature, which self-destructs and renders him comatose, leaving Rex mortified and angry at Ben.

At Rex's hideout, Ben is locked up in a containment unit, while Doctor Holiday explains that the creature was Nanite in origin, but somehow completely different from an E.V.O. Ben offers help but is denied, escaping into the night. They are both confronted by Caesar, who explains the creature's origin. Having, created the creature, whose name is "Alpha". Saying that he created Alpha as a means of manufacturing nanites that could adapt for any scientific need, but became unstable as it developed sentience, forcing [[MadScientist Caesar]] to banish it to another dimension. Alpha invades to absorb E.V.O.'s for their nanites, hoping to gain control of Rex's as it knows he holds the key to it obtaining perfection.

Will Rex and Ben be able to stop Alpha, before they kill each other?

This special is Canon to both, and even introduced a new alien for Ben, Shocksquatch, that was then used in the new series Ben10Omniverse. It was generally well-received by fans of both series, and has hit the top 100 on iTunes.

As this special was semi-livetroped, unmarked spoilers abound.
!!Tropes in the cross-over:
* ActionDuo: Guess who?
* AlternateUniverse: Ben and Rex's Earths are parallel to each other.
* ArbitrarySkepticism: With everything they've gone through, Rex is still resistant to the idea of aliens or parallel worlds. Mitigated by the the fact that (with the exception of that one CryptidEpisode) everything he's seen so far has been a result of terrestrial technology in his own dimension.
** Though it's worth noting that Rex did eagerly consider the possibility of alien technology while being beamed instructions for a machine in "Mixed Signals", incidentally the episode that introduced Caesar.
* AreYouMyMommy: Alpha thinks Cesar is his father. Cesar is less than convinced.
* ArtShift: For Ben and his aliens. They're done in the ''WesternAnimation/GeneratorRex'' (more detailed) style. This has the effect of making Ben look older.
* AsideGlance: Holiday after Cesar takes off with the metal fragment without an explanation. She's probably talking to Six, but is looking right at the audience.
--> '''Holiday:''' For the record, most scientists are not like that.
* AttackOfTheFiftyFootWhatever: [[spoiler: Alpha's final form.]]
* BigApplesauce: Figures Manhattan would be first to get smashed up.
* BigDamnHeroes: White Knight, believe it or not.
* CainAndAbel: Rex and Cesar, to a certain extent. Also, in some twisted way, Alpha considers himself Rex's brother.
* CallBack: Cannonbolt!Ben saves Rex and himself from the exploding hovercraft the same way he protected Gwen and Max from re-entry in [[{{WesternAnimation/Ben10}} the original series]].
** Rex's attempt at creating his own theme song in the opening uses the same tune of the original ''WesternAnimation/{{Ben 10}}'' series with Rex adding his own lyrics. Six admits it's kind of catchy.
** Alpha first act after adapting the Ultimatrix to himself is to turn into his own version of Heatblast. This calls back to Ben's own very first transformation (Heatblast), as well as his first transformation in Alien Force (Swampfire, another fire alien).
* CardboardPrison: Pretty much anything for a guy who can pass through matter, like Ben as Big Chill. (He lampshades this by saying he could have left any time he wanted)
** [[spoiler: Alpha's incredibly dense sphere that Rex created to trap him turns out to be one of these.]]
* CasualDangerDialogue: After Alpha has just gone Heatblast:
--> '''Ben:''' Can he ''do'' that?
--> '''Rex:''' I think he just did.
--> (*''run away''*)
** Later:
---> (*''Alpha has gone Fourarms''*)
---> '''Ben:''' Okay, you got to admit, that looks pretty cool. Should I try those [[YouFightLikeACow insults]] now?
* CompositeCharacter: Alpha is effectively an EVO, and then, at the climax, hacks the Ultimatrix, allowing him to turn into some of Ben's aliens.
* CreateYourOwnVillain: Alpha was created by Ceasar as a way to control nanites. He then sent it into the [[{{WesternAnimation/Ben10}} Null Void]] to try and get rid of it. [[NiceJobBreakingItHero Nice job sending it someplace it could get hosts, Ceasar]].
* DangerRoomColdOpen: When Rex and Six are sparring.
* DoesThisRemindYouOfAnything: '''Alpha:''' "The [[Literature/TheBible beginning and the end]] of all things, I am [=AlphaOmega=]!"
* FastballSpecial: Rex, with Fourarms!Ben as the thrower.
** Rex returns the favor with Cannonbolt.
* FireForgedFriends: when they first meet, Ben and Rex end up fighting each other, and keep arguing for a good part at the beginning of the story, but gradually come to become friends as the episodes goes on.
* FusionDance: Between Rex and Upgrade!Ben. This form comes complete with a super jetpack, RocketPunch (perfect for EyeScream!), MacrossMissileMassacre, a full-on [[CoolBike rocket cycle]] and improved CoolSword.
* GettingCrapPastTheRadar: When Rex and Ben crash back to earth with Rex carried in Cannonbolt:
--> '''Rex:''' ''Ewww!!'' Do...you...have...''any'' idea what it ''smells like'' in there, dude?!
--> '''Ben:''' I'm sure you can handle a little Arborean [Pelarota] musk to save your ''nuts'' and bolts, ''dude''.
* HeterosexualLifePartners: Ben and Rex, by the end of the episode.
** MotifMerger: In the beginning, their respective logos are fused, with the Omnitrix symbol forming the hub of Rex's gear symbol. See again when Fused!Rex builds his Upgrade-enhanced PowerFist.
* HowDoIShotWeb: Ben gets [[ShockAndAwe Shocksquatch]] for the first time, and doesn't know what he does.
* JustEatHim: How [[BigEater Upchuck!]]Ben deals with the super-heavy nanite sphere. Rex's reaction is priceless:
--> '''Rex:''' Wow. That's just... wow.
* LetsYouAndHimFight: Ben thinks Rex is an alien, Rex thinks Ben's an EVO, you get the picture. [[spoiler:It ends in a draw.]]
* NegativeSpaceWedgie: Sets off the plot.
* OneWingedAngel: Alpha does it ''several times'':
** [[spoiler:When he absorbs all nanites in the Bug Jar, this cause him to turn into a massive, bulky, skull-faced being]];
** [[spoiler:When he hacks the Ultimatrix, he is able to steal its abilities and turn into Evos giant versions of Heatblast, Humongousaur, and Fourarms.]]
** [[spoiler:Finally, when he absorbs the Omega nanite from Rex, he is able to turn into an ever bigger and more powerful Evo, calling himself [=AlphaOmega=]]]
* OriginStory: Ben and Rex explain theirs to each other at the same time.
** Also, Alpha's origin is narrated by Ceasar.
* OurVampiresAreDifferent: Like [[GeneratorRex Van Kleiss]], Alpha drains nanites from other things. Only these turn to dust instead of getting TakenForGranite.
* PhlebotinumBreakdown: Alpha manages to knock out Ben's Ultimatrix, so he's reduced to throwing rocks.
* PlayAlongPrisoner: Ben when Providence "captures" him right after their fight.
--> '''Rex''': In case you hadn't noticed, [[TemptingFate we're holding you under house arrest]].
--> '''Ben''': *grins* You aren't holding me. I was staying so I could see how I could help. *turns into Big Chill and walks out*
* PowerCopying: Alpha manages to copy Ben's Ultimatrix, giving him versions of [[PlayingWithFire Heatblast]], [[MultiArmedAndDangerous Fourarms]] and [[EverythingsBetterWithDinosaurs Humongousaur]].
* PsychoPrototype: Alpha the prototype nanite program.
* SentientPhlebotinum: Alpha
* SequelHook: The special ends with many potential elements for a sequel; [[spoiler:Ben is explicitly mentioned to have been infected with nanites upon his arrival in Rex's world, Alpha is shown breaking out of his prison inside the Null Void, and Six suggests Rex and Ben could meet again at the end of the special]]. However...
** AbortedArc: Generator Rex has since then been cancelled, and Derrick J. Wyatt doesn't intend to use any of those {{Sequel Hook}}s.
* TheStinger: [[spoiler:Alpha is showing signs of breaking out of the nanite sphere it's trapped in]].
* TeethClenchedTeamwork: For a while.
* ThemeTuneCameo: Several times, one example being when Ben transforms back to human the ''Ultimate Alien'' theme plays.
* ThreePointLanding: Done by Six early on, when fighting Alpha's original build.
* TransformationNameAnnouncement: Averted in this case. Ben doesn't do it.
* TransformationSequence: Unlike Ben, Alpha gets one for his Heatblast transformation.
* TrappedInAnotherWorld: Ben.
* VeinOVision: Alpha can see the nanites in other creatures.
* WeakButSkilled: Rex compared to Ben. Ben's aliens give him a wider array of abilities and they are on the whole stronger than Rex, but Rex is more creative with the skills he does have. For example, he uses the Slam Cannon to launch concrete to bypass Lodestar's magnetic abilities.
* WhatHappenedToTheMouse: Black Knight is never seen again after her attempt to close the rift fails.
* WorfHadTheFlu: Six was critically injured during the initial fight with Alpha, putting him out of commission for the rest of the special.
** To a minor extent, Ben lost to Rex in their fight only because he was distracted saving civilians, and spent part of the fight with Alpha with his Ultimatrix malfunctionning.