Ben 10/Generator Rex: Heroes United

An Intercontinuity Crossover between the two Man of Action Studios series Ben 10 Ultimate Alien and Generator Rex that aired November 25, 2011.

A mysterious rift begins to open in the skies of Manhattan. Meanwhile, Rex is sparring against, Agent Six. While musing about his need for a "Theme song" before getting the alert about a mysterious rift. Providence appears on the scene to stop the rift, but are unable to do anything before Rex arrives with Six and Bobo. Doctor Holiday, then gets a scan that shows something is coming through the rift, and Rex finds himself (unknowingly) fighting against, 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 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?

As this special was semi-livetroped, unmarked spoilers abound.

Tropes in the cross-over:
"Holiday: For the record, most scientists are not like that."
 * Alternate Universe: Ben and Rex's Earths are parallel to each other.
 * Arbitrary Skepticism: With everything they've gone through, Rex and co. still don't believe in aliens or parallel worlds.
 * Are You My Mommy: Alpha thinks Cesar is his father. Cesar is less than convinced.
 * Art Shift: For Ben and his aliens. They're done in the Generator Rex (more detailed) style. This has the effect of making Ben look older.
 * Aside Glance: 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.

"Ben: Can he do that? Rex: I think he just did. (*run away*)"
 * Big Applesauce: Figures Manhattan would be first to get smashed up.
 * Big Damn Heroes: White Knight, believe it or not.
 * Cain and Abel: Rex and Cesar, to a certain extent. Also, in some twisted way, Alpha considers himself Rex's brother.
 * Call Back: Cannonbolt!Ben saves Rex and himself from the exploding hovercraft the same way he protected Gwen and Max from re-entry in the original series.
 * Rex's attempt at creating his own theme song in the opening uses the same tune of the original 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).
 * Cardboard Prison: Pretty much anything for a guy who can pass through matter, like Big Chill!Ben.
 * Casual Danger Dialogue: After Alpha has just gone Heatblast:

"(*Alpha has gone Fourarms*) Ben: Okay, you got to admit, that looks pretty cool. Should I try those insults now?"
 * Later:

"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."
 * Composite Character: Alpha is effetively an EVO, and then, at the climax, hacks the Ultimatrix, allowing him to turn into some of Ben's aliens.
 * Create Your Own Villain: Alpha was created by Ceasar as a way to control nanites. He then sent it into the Null Void to try and get rid of it. Nice job sending it someplace it could get hosts, Ceasar.
 * Danger Room Cold Open: When Rex and Six are sparring.
 * Does This Remind You of Anything?: Alpha: "The beginning and the end of all things, I am AlphaOmega!"
 * Fastball Special: Rex, with Fourarms!Ben as the thrower.
 * Rex returns the favor with Cannonbolt.
 * Fire-Forged Friends
 * Fusion Dance: Between Rex and Upgrade!Ben. This form comes complete with a super jetpack, Rocket Punch (perfect for Eye Scream!), Macross Missile Massacre, a full-on rocket cycle and improved Cool Sword.
 * Heterosexual Life Partners: Ben and Rex, by the end of the episode.
 * Motif Merger: 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 Power Fist.
 * Getting Crap Past the Radar: When Rex and Ben crash back to earth with Rex carried in Cannonbolt:

"Rex: Wow. That's just... wow."
 * How Do I Shot Web?: Ben gets Shocksquatch for the first time, and per usual doesn't know what he does.
 * Just Eat Him: How Upchuck!Ben deals with the super-heavy nanite sphere. Rex's reaction is priceless:


 * Let's You and Him Fight: Ben thinks Rex is an alien, Rex thinks Ben's an EVO, you get the picture.
 * Negative Space Wedgie: Sets off the plot.
 * One-Winged Angel: Alpha does it several times, ending up as AlphaOmega.
 * Origin Story: Ben and Rex explain theirs at the same time.
 * Our Vampires Are Different: Like Van Kleiss, Alpha drains nanites from other things. Only these turn to dust instead of getting Taken for Granite.
 * Phlebotinum Breakdown: Alpha manages to knock out Ben's Ultimatrix, so he's reduced to throwing rocks.
 * Power Copying: Alpha manages to copy Ben's Ultimatrix, giving him versions of Heatblast, Fourarms and Humongousaur.
 * Psycho Prototype: Alpha the prototype nanite program.
 * Sentient Phlebotinum: Alpha
 * The Stinger: Alpha is showing signs of breaking out of the nanite sphere it's trapped in.
 * Teeth-Clenched Teamwork: For a while.
 * Theme Tune Cameo: Several times, one example being when Ben transforms back to human the Ultimate Alien theme plays.
 * Three-Point Landing: Done by Six early on, when fighting Alpha's original build.
 * Transformation Name Announcement: Averted in this case. Ben doesn't do it.
 * Transformation Sequence: Unlike Ben, Alpha gets one for his Heatblast transformation.
 * Trapped in Another World: Ben.
 * Vein-O-Vision: Alpha can see the nanites in other creatures.
 * What Happened to the Mouse?: Black Knight is never seen again after her attempt to close the rift fails.
 * Worf Had the Flu: Six was critically injured during the initial fight with Alpha, putting him out of commission for the rest of the special.