At a DNAlien lair, one of the DNAliens contacts the Highbreed Commander and informs him that someone has been leaking details of their secret operations to the authorities for several months now. The DNAlien tells him he's found the source of the leak and shows the Highbreed Commander a holographic image of Max Tennyson. The Highbreed Commander orders the DNAliens to go out and destroy Max once and for all so that they can continue with their plans.
Meanwhile, a soccer game is being held at Ben's school. Ben, now 15, is the goalie for his team, while Cash and JT are playing for the opposing team. Cash is in control of the soccer ball and heads towards the goal, but is blocked by three players from Ben's team. There are only 24 seconds left on the clock, so Cash quickly passes the ball to JT, who's open. With less than 10 seconds remaining, JT runs towards the goal and kicks the ball, but Ben manages to block the kick, as the final buzzer sounds. Thanks to Ben, his team wins the soccer game 2-1. The crowd cheers and Ben's teammates pick him up and carry him for helping his team win the game. After the game is over and everyone has left, Ben removes the lock from his bicycle as he prepares to go home. He takes a look at the gold MVP medal he received from the soccer game and feels proud.
Before he leaves, a girl about Ben's age named Julie comes up to him. He introduces himself to her, but she says she already knows who he is and congratulates him on a great game. Afterwards, they say goodbye to each other and Ben rides his bike over to a trailer park where Max's RV is and knocks on the door. Unknown to him, there is a mysterious man hiding in the trees nearby spying on him. Since no one answers the door, Ben looks through one of the RV's windows and sees that the lights are all turned off. He picks up a rock from the ground and grabs the key underneath it to open the RV's door and go inside. As he enters, he sees all of Max's clothes and other items scattered along the floor and wonders what could have happened. He hears a noise coming from the bathroom, so he walks over and opens the bathroom door, but there's no one in there.
As soon as he opens the door, a DNAlien suddenly appears from behind and starts to attack. Ben dodges his attack and instinctively slams down on his wrist, but then remembers that he's no longer wearing the Omnitrix. The DNAlien continues to attack, but Ben is able to continue dodging his attacks. The DNAlien loses sight of Ben for a moment, but spots one of Ben's shoes on the ground. The DNAlien thinks he's found him again, but as he approaches the shoe, he realizes Ben's not there. Ben takes advantage of the situation and comes out from behind the DNAlien, taps it on the shoulder and sprays it in the face with a fire extinguisher, forcing it to escape by jumping out of the RV's front windshield.
Ben wonders what the DNAlien was looking for and flips a few switches on RV's dashboard. As he does so, a secret compartment containing a device pops out from the wall. As soon as Ben grabs it, a pre-recorded holographic video of Max starts playing. In the recording, Max tells him that he's in a bit of a situation, but that it's nothing he can't handle, so Ben shouldn't worry. He continues to say that there is renewed alien activity on Earth and that he's investigating. Max also says that he has the Omnitrix and that it is completely safe and that the aliens will never find it. After listening to Max's recording, Ben rushes over to his house and goes straight to his room to see if what Max said about the Omnitrix is true. He opens a cardboard box and finds the Omnitrix still in there, leaving Ben confused as to what Max was trying to tell him.
At the school gymnasium, Gwen, also 15, is having a karate match against another student. As soon as the match begins, Gwen quickly takes down her opponent and wins the match. The other students clap as Gwen and her opponent bow to each other. As the other students stop clapping and walk away, Ben shows up and claps for Gwen, as well. She walks over to him and hugs him. Ben tells her that he needs her advice, but first she tells him she needs to go change into her regular clothes first. When Gwen returns, she finds Ben sleeping on the floor. He wakes up and tells her he needs advice and holds up the Omnitrix. He then shows her the recording Max left him. Ben thinks Max is telling him to put on the Omnitrix again, but Gwen tells him that Max has always said it was his choice if he wanted to put it back on. Ben says he used to love wearing the Omnitrix, since it made him feel special. But Gwen tells him it wasn't the Omnitrix that made him feel special. She warns him that if he puts it back on, it'll be the end of his normal life. Ben thinks that Max needs his help, but Gwen tells him Max told him he was safe in the recording.
While they're talking, the mysterious man from before enters the gym and tells him that the Omnitrix doesn't belong to Ben and wants him to give it to him. Ben and Gwen run towards the exits to try to leave the school, but the doors are locked. The mysterious man catches up to them, asking for the Omnitrix, and reveals his true identity as an alien being named Magister. Ben tries to run, but Magister stops him by shooting his laser gun. To protect herself and Ben, Gwen uses her magic spells to attack Magister and manages to capture him in an energy hold. Ben compliments her on getting better with her magic skills. Ben wants to know why he's attacking them, and Magister replies by saying he's a member of the Plumbers. Ben tells him his grandfather Max used to be a Plumber. Magister tells them Max was working with him on a case, but went missing. Gwen tells him that they're also looking for Max. Magister then realizes that Ben is the "legendary" Ben Tennyson. He attacked them earlier because he thought he was just some kid stealing the Omnitrix and apologizes to them. Magister promises to help them look for Max, so Gwen releases him from the energy hold.
Ben takes out the Omnitrix and thinks it over one last time. Gwen asks him if he's sure, and Ben puts it back on and tells her that he is. Afterwards, Ben, Gwen and Magister go to an abandoned train station, where the Forever Knights are supposed to receive a shipment of illegal alien technology, according to a tip from Max. Gwen asks Magister who they're getting the shipment from, but he says he doesn't know, since Max went missing before he could tell him. As they hide behind some nearby crates, two shipment trucks arrive, followed by a green muscle car. Kevin steps out from the green car, and Ben and Gwen instantly recognize him. One of the Forever Knights gives Kevin a briefcase full of money, and Kevin signals one of his companions to unload the alien technology from the truck. Kevin opens one of the briefcases containing one of the alien weapons called a Laser Lance. As proof that it works, he fires at one of the empty trains nearby.
Magister becomes infuriated at the fact that the Laser Lances are level five technology, while Earth is only supposed to have level two. Magister walks over to them, followed by Ben and Gwen, and tells them they are all under arrest. However, the humans supplying the alien weapons transform into DNAliens, so Magister fires his laser gun along the ground, creating a small fire between them so they don't get too close. But Ben then tells him that they are also surrounded by the Forever Knights behind them. Ben tries to transform to battle them and slams down on the Omnitrix, but it doesn't work, as the display has turned blue. He tries turning the dial, but it still won't work. So Gwen uses her magic to push back the Forever Knights, but they get back up again start firing their Laser Lances at them, so Gwen puts up a shield using her magic spells.
Magister gets captured by the DNAliens' spit and is unable to move. Ben is then chased by one of the DNAliens, but is able to stop him by dumping a pile of wooden sticks on him. He then tries to get the Omnitrix to work again. Gwen pushes back a few of the Forever Knights, but ends up getting stuck in the DNAliens' spit and becomes unable to move, like Magister. Meanwhile, the Omnitrix transforms and reshapes itself on his wrist. Ben turns the dial on the Omnitrix and a hologram of the different aliens is displayed, but Ben doesn't recognize any of the aliens. So he chooses one at random and transforms into Swampfire, a smelly plant-like alien that can shoot fire and has the ability to regenerate itself.
Swampfire easily defeats a few of the DNAliens, but two of them try to escape by driving away in the truck. However, Swampire rips out a metal post from the ground and hits the truck with it, preventing them from escaping. The Forever Knights try shooting their Laser Lances at him, but the lasers have no effect on him, as they just go right through him. However, one of the lasers ends up cutting off Swampfire's right arm, but thanks to his powers, he's able to reattach the severed limb back on his body. Swampfire punches away the Forever Knights, but Kevin steps in to confront him about trapping in the Null Void five years ago and for ruining his deal with the Forever Knights, so now he wants revenge. He absorbs some of the metal from the railroad tracks and makes his body completely metallic. While Swampfire and Kevin fight, the Forever Knights drive away with their shipment of Laser Lances and the DNAliens escape with the money.
To try and defeat Swampfire, Kevin picks up a train cart, but before he's able to throw it, Swampfire shoots fire at it, causing it to explode, leaving Kevin unconscious. When he wakes up, he finds himself bound by energy cuffs, so he can't use his absorbing powers. Ben tells Kevin that the Forever Knights got away with the Laser Lances. Since Kevin set up the deal, Magister wants him to tell him where they are. And since he never got paid, Kevin agrees to help them find the Forever Knights and they all head for the Forever Knights' headquarters in Kevin's car. Magister tells Ben that Max thought the weapons deal would lead them to the cause of the renewed alien activity on Earth, but then he disappeared. Gwen jokes that he's probably in a restaurant somewhere, enjoying some weird food, but Ben yells at her to stop making jokes about him. Kevin immediately stops the car and tells Ben not to yell at her and apologizes to her. Gwen understands that he's worried about Max, but Magister tells Ben that he's going to have to figure what to do on his own, since Max isn't here.
They finally reach a large castle, where the Forever Knights are supposed to be. Gwen uses her magic to forcefully lower the drawbridge so they can get in. Since no one is inside, Gwen asks Kevin if they're at the right place. He tells them be on the lookout for a hidden door. Ben looks around and sees a lot of empty medieval armor suits, similar to the ones the Forever Knights wear. He accidentally bumps into one and barely manages to avoid getting hit by it. However, the sound of the armor crashing on the floor wakes up a huge dragon creature, ready to attack them.