Ben, Gwen and their paternal grandfather Maxwell Tennyson, spend their evenings fighting aliens on the streets of Bellwood. During a battle that involves chasing and fighting a robotic tank, Ben as Upgrade and Gwen argue about the best way to defeat it, resulting in Gwen utilizing her extraordinarily strong magic to cast a dismantling spell on the tank while Ben is still attached to it. Though this does defeat the tank, unknown to the group, it leads to a malfunction in the Omnitrix.
It is later revealed that the new ability prevents Ben from returning to human form when the Omnitrix times out, simply switching into another one of his alien forms. Ben as Four Arms arrives at Stonehenge, but is confronted and attacked by a Galvanic Mechamorph (Upgrade's species), with abilities of forming weapons and armor from its body and teleportation, who demands to know about the To'kustar that had attacked Ben. Ben explains to the Mechamorph that he knows nothing, but it only continues to attack, and its teleportation ability causes the pair to carry the rest of their battle across the globe: after destroying part of Stonehenge, they nearly damage Christ the Redeemer in Rio de Janeiro and fight at the Great Pyramids in Egypt before arriving back in Bellwood. Ben transforms into Grey Matter and manages to escape the Mechamorph.
Meanwhile, Gwen and Grandpa Max receive a call from Ben's parents informing them that Ben is missing. They go in search of him only to find Tetrax. The three discover Azmuth's ship, but with no sign of Azmuth. Looking at the ship's log, they discover that Azmuth disguised it as a truck to track down and fix the malfunctioning Omnitrix, but crashed and was destroyed by the To'kustar. After escaping the Mechamorph, Ben as Grey Matter is mistaken as a small creature by his parents before Gwen, Grandpa Max and Tetrax soon encounter him back at his house. Tetrax restores Ben's Omnitrix to normal just as the Mechamorph catches up to them. Gwen deduces how the Omnitrix reacts after Ben returns to his human form and realize what has happened just as she, Ben, Tetrax and the Mechamorph are transported into the Omnitrix itself.
Now regaining his senses, Azmuth proceeds to use his father's Mechamorph abilities to fix the Omnitrix and subsequently allow it to return himself to normal. Azmuth reprimands Ben and Gwen for their errors and reveals that his father is actually a Galvan like him, wearing a Mechamorph-type enhanced armor that, as Azmuth reprimands him for as well, amplifies any anger of the user explaining his behavior. Hearing Azmuth explaining of how the Omnitrix was so filled with pink mana, Gwen wonders just what he is referring to, with Azmuth telling her that she will find out eventually. Returning to Ben's house, Azmuth restores Ben's parents to normal, albeit unconscious and their memories of grounding their son lost due to the effects of the transformation, and repairs the Rust Bucket, previously destroyed in the battle, before leaving to reconcile with his own father.
Later, Ben is seen riding on his hoverboard through a green-colored Bellwood, which he finds abandoned. He stumbles across a figure at a dark corner who he believes to be Max. The figure ignites, revealing itself to be Heatblast, and attacks Ben. Ben escapes, only to then be attacked by Four Arms, Stinkfly, Diamondhead, Wildmutt, and Upgrade. Ben manages to get away from all of them, but the ground collapses and he is grabbed by Way Big who demands his homework. Ben wakes up to find his teacher asking the same question. Ben tries to submit the homework into his teacher, but the moment his teacher touches it the Omnitrix flashes and the homework disappears. The class is dismissed and the teacher assigns Ben a history report. When the teacher grabs Ben's planner and looks through it, he finds an unflattering picture of himself in it, due to which Ben ends up in detention. At his locker, Ben laments that he isn't going to save the world anymore when Cash and J.T. appear and close him in his locker. Ben attempts to transform into Four Arms, but he transforms into Heatblast instead. Heatblast then breaks out of his locker, causing the fire alarm to go off, and flies home. While inside of his house, Heatblast hides in his bedroom when his parents Carl and Sandra arrive. Fortunately for him, Heatblast reverts back into Ben just in time, but his parents noticed that there are burnt footprints on the carpet leading to his room and ground him when he can't come up with an explanation. Ben states that he is supposed to go fishing with Grandpa Max, but his parents stick to the punishment, as they believe that missing one trip with his grandfather won't be a big issue since he spent the whole summer with him. Afterwards, Ben calls Max in the hopes that he's needed to save the Earth and reveals he wishes he was still on summer break fighting aliens around the country, but Max won't help him as he doesn't want Ben to push away his family like he almost had with Ben's father when Max was still fully active as a Plumber.
Meanwhile, in the Rust Bucket, Gwen tries and fails to track Ben, only getting as far as the forest. They search through the forest and perceive Tetrax's crystals and the abandoned truck. Gwen casts a spell on the truck and reveals it to be a ship. Afterward, Tetrax's ship lands on the ground and Tetrax begins to speak to Max and Gwen. As Diamondhead is walking down the street, the Omnitrix times out and Diamondhead transforms into Four Arms instead of reverting back into Ben. Four Arms arrives at Stonehenge and he then is attacked by a Galvanic Mechamorph named Retaliator. Retaliator says that he is looking for an alien and Four Arms and Retaliator engage in a fight, but Retaliator can teleport. Stonehenge starts to topple, but Four Arms catches it and Retaliator says that it is a Galvan practical joke. Retaliator grabs Four Arms, but Four Arms throws him off and Retaliator morphs into a mechanical warrior. Retaliator keeps demanding an alien and Four Arms knocks him away, but he teleports back and attacks Four Arms. His missiles carry Four Arms into the sky, but he figures out how to control his flight and flies back to Retaliator. Retaliator flies to meet him and shoots him, but Four Arms throws the missiles at Retaliator and they cause an explosion. Four Arms jumps onto Retaliator and they crash into the ground. Retaliator says that he shot down Tetrax's ship and blasts Four Arms away. Four Arms mentions Azmuth hand Retaliator asks what he knows of him. Four Arms and Retaliator keep fighting as Retaliator continues to demand information. They destroy Stonehenge and Four Arms swats Retaliator away before he transforms into Upgrade again. Retaliator appears and is confused to see Upgrade and he asks where Four Arms went. Upgrade pretends to not know, but Retaliator decides to interrogate him. They teleport away and end up in Rio. Retaliator says that Upgrade caused his teleporter to malfunction and then he shocks Upgrade and interrogates him. They teleport again and land on the pyramids on Egypt. Retaliator says that they were made by Tetramands, but Upgrade transforms into Stinkfly and escapes from Retaliator's grasp. After attaching Retaliator to the pyramid with his slime, Stinkfly explains the Omnitrix, but Retaliator doesn't believe him. Retaliator chases him and shoots Stinkfly, which knocks him out of the sky. Retaliator and Stinkfly then teleport into Bellwood and land on a building. Meanwhile, in the forest, Tetrax explains what happened to Max and says that he altered the Omnitrix to keep Ben from reverting back into normal. Max is furious about what has happened and Gwen says that Azmuth might be able to find Ben and says that the ship is his. They enter into the ship while Retaliator chases Stinkfly around the city and shoots at him. Retaliator projects water at Stinkfly that knocks him into a building, but Heatblast emerges from the building and flies to attacks Retaliator. Retaliator freezes Heatblast and grabs him with giant tweezers. While dangling him off of a building. Retaliator asks Heatblast where to find the To'kustar. Heatblast says that he disappeared, but Retaliator doesn't believe him. Luckily, Heatblast transforms into Grey Matter and escapes from Retaliator's grasp.
They fly back to the battle and Gwen thinks that magic and Upgrade caused the problem, so they can also fix it. Gwen intends to use Retaliator, who is fighting the To'kustar with Tetrax, instead of Upgrade. The To'kustar falls down and Gwen casts a spell on Retaliator, which causes him to bring everyone out of the Omnitrix. After Max tries to fix in the Rustbucket, he makes it outside just in time as the Rustbucket is destroyed by the To'kustar. Ben sees the Omnitrix on his wrist and realizes that Gwen's plan worked. The To'kustar stomps off and Retaliator follows. When Ben tries to transform, the Omnitrix flashes and projects a blast at Carl and Sandra, transforming Carl into Wildmutt and Sandra into Heatblast. Wildmutt attacks Ben while Heatblast attacks Max and Gwen. Retaliator attacks the To'kustar, but it throws him into a building. Tetrax distracts Wildmutt and Gwen saves Ben from Heatblast. Tetrax says that Ben's parents are disoriented and Gwen realizes that the To'kustar must be Azmuth due to the fact that the Omnitrix transformed him in the same way as Ben's parents. Ben realizes that she is right and Tetrax tells Ben and Gwen to stop Azmuth and Retaliator. Ben transforms into Way Big and he grabs Gwen and walks to where Retaliator and Azmuth are fighting. Way Big grabs Azmuth and pulls him off of Retaliator. Way Big and Azmuth fight and knock down buildings. Gwen shields herself as the To'kustars fight and Way Big knocks out Azmuth, but Retaliator sees him and attacks. Way Big is sent flying and Gwen tells Retaliator that Way Big is Ben. Way Big explains that the other To'kustar is Azmuth, but Retaliator doesn't believe