It starts out in the BTMT crosstime. Ben and Co. were driving in Kevin's car toward Ben's house. Kevin (of course) was driving.
(Ben): Guys, I need something to do.
(Gwen, sarcastically): Wow. It's hard to find that!
Kevin dropped Ben off at his house, and drove away with Gwen toward her house.
Meanwhile, as Gwen and Kevin rounded a corner in the car, they crashed into a glowing pink wall! The car slammed through the wall, and it got really beat-up from the impact.
(Kevin): Hey, watch the paint jo...!
He stopped short as he saw the two people behind where the wall were: themselves!
(Gwen): Someone cloned us!
(Other Gwen): You have no idea how wrong you are!
Gwen and the other Gwen shot mana at each other. The beams cancelled each other out, yet knocked out Gwen. The other Kevin knocked out normal Kevin easily.
(Other Kevin, with an evil smile): Nighty night!
Back at Ben's house, Ben was sitting in the couch, reading. Azmuth suddenly teleported in.
(Azmuth): Ben! I need you to...
Suddenly, Ben's mom, Sandra, came in, and screamed upon seeing Azmuth.
(Sandra): Eek! A rat! Get it out! Get it out!
(Azmuth): I am not a...
But Sandra already smacked Azmuth with a rolled-up newspaper.
(Azmuth, under the paper): *sigh* How did I not see this coming?
(Ben): Mom! That's Azmuth!
(Sandra): That thing is the creator of the Omnitrix?
(Azmuth, still under the paper): I beg your pardon.
(Sandra): I'm sorry.
She picked up the paper, and left. Azmuth got up.
(Azmuth): *cough!* As I was saying, you must save your cousin and Levin. They have disappeared!
(Ben): What? How?
(Azmuth): I can answer that.
He jumped to the All-Power-Trix and used its communicator function.
(Azmuth, into it): Okay, Paradox.
He hung up. Then, Paradox suddenly teleported in... with another Ben! He had a different Omnitrix on.
(Ben): WHAT IS GOING ON HERE?
(Paradox): Allow me to introduce you to Ben Tennyson, from Crosstime Path Number 59,427,108.
(Ben): Ugh. Not another crosstime version of me. One is enough.
(Paradox): I told you. There are infinite Crosstime paths, and hundreds of realities that each have their own set of paths.
(Other Ben): Let me clear things up. A Techadon's bomb exploded in my Crosstime, turning all heroes evil, and all villains good. The good news, my Omnitrix kept me good. The bad news, Albedo's Omnitrix kept him evil. Azmuth soon gave me a new Omnitrix. Sadly, Gwen and Kevin turned evil in the whole thing. They must have come to this crosstime somehow. We must stop them!
Meanwhile, Gwen and Kevin were being held in a dark room by their mysterious evil counterparts. They were chained to the wall.
(BTMT Gwen): Who are you? Clones?
(TF Gwen): No.
(TF Kevin): Yeeeeeeeeeeeeeeeeeaahh... no.
(TF Gwen): Now tell us the secrets to the All-Power-Trix.
(BTMT Gwen): Why should we?
TF Gwen's hands glowed, but not with mana. They were sparking up with electricity!
(BTMT Gwen): Since when can I fire electricity?
(TF Gwen): You can't! I, on the other hand, practice regularly and happened to discover this!
(TF Kevin): We're gonna say this one more time: tell us the secret, or be dumped in the Null Void!
(BTMT Kevin): No!
(TF Gwen): Have it your way. You leave us no choice.
She pushed a button on a control panel, and a huge portal opened up! The panel was sucked in, but the force of the suction wasn't strong enough to pull BTMT Gwen and Kevin out of their chains. TF Gwen and Kevin were behind the portal.
(TF Kevin): Remember: our Null Void is a lot tougher than your wimpy one!
Suddenly, BTMT Gwen and Kevin's chains snapped! They were almost pulled in, screaming, when BTMT Ben (as Four Arms) jumped out of nowhere and pulled them aside. TF Ben used his Omnitrix to suck up the energy of the portal into its battery, closing the portal.
(BTMT Ben): You are not dumping my cousin and friend in the Null Void!
(TF Ben): Technically, it was my Null Void.
(BTMT Ben): Whatever. You're about to taste some real fighting power! (Transform) RATH! (Transform again) ULTIMATE RATH!
He charged at TF Gwen and Kevin, but TF Gwen put up multiple shields, then sandwiched them together, making a thicher shield that was nearly indestructible. BTMT Ben just clawed at it, scratching it, but not doing anything else.
(TF Ben, transforming): Georock!
He started punching the shield, but only chipped pieces off. It also chipped pieces of himself off, too, showing that Georock isn't really 100% indestructible.
(TF Ben): This isn't doing anything! Gah! (Transform) Chromastone!
He tried absorbing some shield energy, but it overwhelmed him quickly. He backed up a few steps and stood there, glowing multiple colors, in a daze. As for BTMT Ben, he was still clawing at the shield, but was getting tired, and was panting. Now he just looked funny. TF Kevin suddenly jumped over the shield, now made of concrete, and punched TF Ben. This made him come to his senses, and he shot some of that energy at TF Kevin, who absorbed it. TF Kevin just stood there, dazed, for a while. Suddenly, he came to his senses, and the concrete shattered.
(TF Kevin): Huh? (realizes what's going on) Oh, right.
He absorbed a hole right through the shield and punched TF Gwen.
(TF Gwen): Kevin! What are you doing?
(TF Kevin): Absorbing energy when I was good turned me insane. Now that I'm evil, it just turns me good.
He punched her again, knocking her out. He then walked over to the almost-ready-to-faint BTMT Ben. He detransformed Ben, who then fell over. A few seconds later, he came to his senses.
(BTMT Ben): Huh? (fully wakes up) You! (prepares to transform)
(TF Kevin): Wait a minute! I absorbed some energy, and it turned me good, since I was already evil. I'm on your side now!
(Paradox, teleporting in): Good show! However, it seems it is over.
He teleported them all to Azmuth's lab, where he was working on a machine with two helmets connected to it by a wire. TF Gwen was still knocked out, and TF Kevin was still good.
(Azmuth): It's about time!
(TF Ben): My crosstime's Kevin is finally cured!
(TF Kevin): Not permanently. I can already feel the energy... fading... (faints)
(Azmuth): He's about to turn evil again! Better hurry! This machine will turn the other Gwen and Kevin permanently good.
(TF Ben): Finally!
He put the helmet on the unconcious TF Gwen and Kevin. He started firing up the machine, when TF Kevin woke up.
(TF Kevin, angry): WHAT ARE YOU DOING?
(TF Ben): No!
(TF Kevin): Turning me good? No way!
He ripped the wire for his helmet, and pulled the helmet from TF Gwen as she started waking up.
(TF Ben): Stop!
TF Kevin, ignoring him, smashed the machine.
(TF Ben): NO!
Suddenly, TF Azmuth teleported in.
(TF Azmuth): It seems my crosstime's Ben is missing. I'm here to take him home. Paradox, please tell me before you just take my crosstime's last hope, okay?
He shot TF Gwen and Kevin with a Galvan-sized Null Void Projector, and then teleported himself and TF Ben home.
(BTMT Ben): Well... that's that.
(BTMT Gwen): Yeah. I hope there's not even a minuscule crossover like this for a long time.