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 manufacturing nanites that could adapt for any scientific need, but became unstable as it developed sentience, forcing Caesar to banish it to another dimension. Ben surmises it may have been the Null Void where it wound up, and that Alpha must have absorbed alien life to develop its current form. Caesar explains that the damaged ray gun was to stop Alpha, who then attacks and destroys his flying lab. Ben saves Rex by becoming Cannonbolt, and the two return to base when Alpha invades to absorb the EVOs inside for their nanites, hoping to gain control of Rex's as it knows he holds the key to it obtaining perfection. 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 second-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 that called 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 most of his 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, and Rex uses the Upgrade Suit 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
 * Gwen Tennyson (Flashback only)
 * Max Tennyson (Flashback only)
 * Kevin Levin (Flashback only)
 * Gabriel Rylander (Flashback only)
 * Van Kleiss (Flashback only)
 * Diane Farrah
 * Diane's Camera Man

Villains

 * Alpha
 * Alpha/Omega (Evolution)

By Ben

 * Humungousaur
 * Diamondhead (x3)
 * Lodestar
 * Rath (x2)
 * Big Chill (x3)
 * Cannonbolt (x2)
 * XLR8 (first reappearance since the original series)
 * Four Arms
 * Shocksquatch (debut)
 * Upgrade (first reappearance 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 transformed into Upgrade, the Ultimatrix symbol wasn't there at all.
 * When Ben and Rex were on the building, Lodestar's mouth was open when the billboard started to fall.
 * 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, Goop's anti-gravity disk is gone, also 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, Wildmutt is missing teeth in the middle and Heatblast looks like the one in the original series because the rocks on his body don't seem as dark and the flames look less bright although the eyes are not connected to his head unlike the original series,AmpFibian's face/head pattern is different, the zigzags on Jetrays body are straight lines instead.
 * 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 the flashback, Rath's fur is gone, while when not in the flashback, his fur is there.
 * 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 the flashback, Ben twice implied that the DNA is inside it. But it's really inside the Codon Stream on Primus. So in other words it's connected to a server.

Trivia

 * This special is the second Cartoon Network crossover after the 2007 crossover special: The Grim Adventures of the KND.
 * Rex confirms that Ben's jacket is very tight. However, it should be noted that Rex is noticeably taller than Ben which also explains why Rex's jacket looked bigger on Ben.
 * However, Ben said that Rex's jacket smells like bananas. It is unknown why.
 * Ben and his alien counterparts are animated in the art style of Generator Rex.
 * In the flashback, if you look in the background, you see Way Big's head.
 * The crossover is "a part of Generator Rex's regular production cycle".
 * The lake where Rex was thrown by Humungousaur resembled the symbol of the Ultimatrix.Ultimatrix_like_lake.png
 * Diamondhead turns his arm into a diamond cluster when fighting Rex that is similar to a toy part coming with the new Diamondhead action figure.
 * This is the first episode since the original series where Ben doesn't call out any of his known alien's names. The only reference to the names was when Humungousaur said "You just made a HUMUNGOUS mistake!", when Rath said "The Rath attack." and when Ben said "You get an Upgrade."
 * Gwen, Kevin and Grandpa Max appear in flashback as well as a variety of Ben's aliens. A flashback of 10-year-old Ben was also shown.
 * There is no transformation sequences of Ben's aliens, however there is a transformation sequence for Alpha Heatblast.
 * Ben seems to revert back to human form, then transform into another alien right after being captured by Providence.
 * The Null Void is seen at the end of the special, hinting at Alpha's return. Ben's World was also revealed during Ben's Flashback.
 * None of Ben's Ultimate Forms appear in the special, even though three aliens that do appear (Humungousaur, Big Chill and Cannonbolt) have Ultimate versions.
 * AmpFibian is the only alien in the flashback to be from the Andromeda 5.
 * Alpha's form, color and constant need of energy is similar to P'andor's.
 * When Alpha nanite takes the Omega, Rex says "My most powerful builds came from the Omega nanite. A lot of people are going to get hurt and I can't stop it. What happens when the best you can do just isn't enough.", Ben says "You get an upgrade". This parallels to when Ben got his Omnitrix taken from him by Vilgax and he had to get "an upgrade" by taking the Ultimatrix from Albedo. However, here Ben is talking about using Upgrade.
 * Rex's "theme song" sounds extremely similar to the one sung in the original Ben 10 series.
 * Rex also shortened Generator Rex in the name of the show to Gen Rex so the song can sound alike.
 * Since the original Ben 10 series, the crossover episode has shown the most transformations Ben has ever made into the original 10 line-up in a single episode.
 * When Caesar ejects Ben and Rex from the ship and Ben returns to human form from Cannonbolt he mentions Cannonbolt's Species.
 * Ben gained nanites from his time in Rex's world. Also if you notice the scene where Rex was trying to cure him he injected his nanites and was left in their by accident in their battle. Also the nanites are inactive at this time.
 * the Title might have been based on the Superman/Batman Heroes United mobile game.
 * The especial is seen in one part of Ultimate Alien, but in The ultimate enemy Ben have a new omnitrix.