Ben 10-Generator Rex: Heroes United

Ben 10-Generator Rex: Heroes United is a crossover episode animated in Generator Rex's art with an appearance of Ben Tennyson.

Plot
The story begins with a mysterious rift opening in the skies above New York City. Meanwhile, Rex is sparring against Agent Six, while musing about his need for a "Theme song" (and briefly performing a take on the original Ben 10 theme song) before getting the alert about the rift. Providence appears on the scene to stop the rift, but are unable to do anything before Rex arrives with Six and Bobo. Holiday then gets a scan that shows something is coming through the rift, and Rex finds himself face to face with Humungousaur, and the two begin battling it out in Central Park. During their skirmish, a strange metallic creature also appears and heads into the city, with Bobo and Six on its trail while Rex continues to battle Humungousaur, who changes into Diamondhead, much to Rex's shock. Rex is also troubled by the fact that his ability to cure has no effect on his opponent.

Meanwhile, Six engages the creature and manages to cut a piece of it off, which is then taken by Rex's brother, Caesar. Rex and Diamondhead then wind up on the Brooklyn Bridge, where the latter changes into Lodestar before the fight takes to the rooftops. However, a falling Billboard forces the two to stop fighting, with Lodestar becoming Rath to save news reporter, Diane Farah. Rex then literally gets the drop on Rath and once again attempts to cure him of his nanites, only for Rath to change into his true form, Ben Tennyson. Ben is shocked to learn that no one knows about him. Just then, Six appears with the creature, which self-destructs and renders him leaving Rex mortified and angered.

At the hideout, Holiday explains that the creature was Nanite in origin, but somehow completely different from an EVO. Ben, who appears to be contained by Rex, escapes as Big Chill after failing to gain the group's trust, and flies off with Rex in hot pursuit. The two clash, while being tailed by a strange ball of red energy. After visiting the area where Bellwood should be, and finding that himself, Gwen, Max, and Kevin do not exist in this world, nor does Mr. Smoothy (instead that's "Bob's Biscuit Barn"), Ben finally comes to grasp that he must be in an alternate dimension, and Rex and he finally come to a truce. Cesar then appears and seemingly prepares to attack the two with a ray gun, which was actually intended for the energy ball, which is revealed to be the Alpha Nanite, a creation of Caesar's.

After escaping the creature, Caesar explains that he created Alpha as a means of controlling the nanites during the Nanite project, but it's nanite form is unsustainable and every mechanical body it built burnt out in short amount of time. Alpha attempted to take organic body forcing Caesar to banish it to another dimension.

Ben surmises it must have been the Null Void where it wound up, and that Alpha must have possessed alien life to sustain itself. Caesar explains that the damaged ray gun was to stop Alpha, who then attacks and destroys his research pod. Ben saves Rex by becoming Cannonbolt, and the two return to base when Alpha invades to absorb the E.V.Os inside for their nanites, hoping to gain control of Rex's as it knows he holds the key to it creating a sustainable body. Ben and Rex eventually manage to drive Alpha off, and then slowly bond after sharing their origins during a small game of basketball.

White Knight then reveals that Alpha has gone to the Bug Jar, which is the largest concentration of active nanites on the planet after the Nanite Event. Rex, White Knight, and Ben arrive, only to find they are too late to stop Alpha from absorbing the nanites of all the EVOs that were in the Bug Jar to become a colossal giant that began to attack the three of them. After an intense battle, Alpha begins to attack Rex in order to take control of his Omega Nanite. During the battle, Ben gains the form of Shocksquatch, but then temporarily loses his powers when Alpha hacks into the Ultimatrix through the inactive nanites that Ben absorbed earlier in order to create his own corrupted version that allows Alpha to take on Nanite powered versions of Heatblast, Four Arms, and Humungousaur. After a tough battle, Rex is able to disable Alpha's imitation Ultimatrix powers, but is ensnared by his enemy and robbed of his Omega Nanite, leading to Nanite Alpha to transform into a colossal crab-centaur-like being calling itself Alpha-Omega; the beginning and the end.

As the now nearly unstoppable Alpha-Omega rampages through the Bug Jar; absorbing the ruined structures in order to create new nanites for itself, Rex has doubts about himself now that his most powerful builds are gone. Ben however comes up with a plan, and transforms into Upgrade and merges with Rex, "You build stuff, I make it better. Fight fight fight, we win." The duo then take on the Alpha-Omega, weakening it enough for Rex to finally condense Alpha into a sphere of matter that weighs several tons. While this happened, Caesar appears and extracts the Omega Nanite. Attempting to dispose of Alpha, Caesar then opens a rift for Ben, who changes into Upchuck and swallows the compacted Alpha as he departs to the Null Void and back to his own Earth, but not before weirding out Rex one last time.

In the epilogue, Rex is overjoyed that Six has recovered, and wonders if he'll ever see Ben again. Six suggests that they possibly will, if fate had ordained for them to meet once already. Caesar appears and reinjects the Omega Nanite into Rex, who fears that it may actually be the Alpha Nanite, although Caesar assures him that there is a 99.998% chance that it is Omega. Finally, in the depths of the Null Void, Alpha begins to stir within its prison of matter, causing it to glow and crack apart.

Major Events

 * Ben travels to an alternate universe, meets Rex Salazar, and meets other characters of his Universe.
 * Ben fights with Rex against Alpha.
 * Ben transforms into Shocksquatch for the first time.
 * Two of Ben's aliens in the original series return. (XLR8 and Upgrade)
 * Ben turns into Upgrade and merges with Rex.
 * Alpha absorbs Rex's Omega nanite and becomes Alpha-Omega.

Characters

 * Ben Tennyson
 * Rex Salazar
 * Rebecca Holiday
 * Caesar Salazar
 * Bobo Haha
 * White Knight
 * Black Knight
 * Agent Six
 * Diane Farrah
 * Diane's Camera Man

Flashback Characters

 * Gwen Tennyson
 * Max Tennyson
 * Kevin Levin
 * Gabriel Rylander
 * Van Kleiss
 * Raphael Salazar
 * Violeta Salazar

Villains

 * Alpha
 * Alpha/Omega (Evolution)

By Ben

 * Humungousaur
 * Diamondhead (x3)
 * Lodestar
 * Rath (x2)
 * Big Chill (x3)
 * Cannonbolt (x2)
 * XLR8 (first re-appearance since the original series)
 * Four Arms
 * Shocksquatch (first appearance)
 * Upgrade (first re-appearance since the original series)
 * Upchuck

Flashback only aliens

 * Stinkfly
 * Echo Echo
 * Goop
 * Chromastone
 * Ripjaws
 * Spidermonkey
 * Brainstorm
 * Wildmutt
 * Heatblast
 * AmpFibian
 * Way Big

By Alpha

 * Alpha Heatblast
 * Alpha Four Arms
 * Alpha Humungousaur

Rex's Machines Used

 * Smack Hands
 * Blast Caster
 * Slam Cannon
 * The B.F.S
 * Boogie Pack
 * Battle Axes
 * Upgrade Suit (first appearance for Rex)
 * Rex Ride
 * Punk Busters
 * Block Party

Errors

 * When Ben and Rex were on the building, Lodestar's mouth was open when the billboard started to fall.
 * When Ben transformed into Upgrade, the Ultimatrix symbol wasn't there at all, but when Upgrade merged with Rex, the Ultimatrix symbol appeared.
 * When Big Chill was talking to Rex after escaping from Providence the inside of Big Chill's eyes were light green and the outside was dark green, but when he froze Rex's fist the inside of his eyes were dark green and the outside was light green.
 * In one scene, Diamondhead's head has an error.
 * When Rex is first holding Six his glasses are off and his eyes are closed, then when Rex tells Ben it was all his fault Six has his glasses on.
 * In one scene, Rath has five fingers.
 * In the flashback, Stinkfly's Ultimatrix symbol is on his head (though since it is his original design it could be argued if this was intentional), both Echo Echos have different lip colors, Big Chill's wings are gone and so is his chest plate, Chromastone's mouth is green, Upgrade's lines aren't glowing, one of the eyes of Ripjaws is pink while the other is green, Diamondhead's spikes are missing, the dashes coming off of Cannonbolt's eyes were green instead of black, Spidermonkey has no arms, Brainstorm is tiny, and Wildmutt is missing teeth in the middle.
 * In the flashback, Rath's fur is gone, while when not in the flashback, his fur is there.
 * In the flashback, Ben twice implied that the DNA is inside the Omnitrix\Ultimatrix. But it's really inside the Codon Stream on Primus. So in other words it's connected to a server. Which makes it a little strange how the Ultimatrix can connect to a planet that's not even in the same dimesion.
 * When Ben reverts from quite a few aliens in this crossover, the sound of the Ultimatrix being activated and then hit down is played.
 * In the flashback, Ben says that Kevin, Gwen and he are the Plumbers and did not mention that it is not just them who are the Plumbers. Also, even though Grandpa is shown in the picture of the Plumbers, Ben does not mention him.

Trivia

 * Rex's "theme song" is taken from the theme song of the original Ben 10 series.
 * The Ultimatrix symbol is shown to be a glowing yellow (instead of green) and black throughout the episode.
 * Ben does not shout the names of any of his transformations in this crossover.
 * After XLR8 saves Holliday, he could have easily transformed straight into Four Arms. It is unknown why he didn't do this.