Underground, the Forever Knights accidentally break the seal that was imprisoning to a legendary and dangerous creature of another dimension. The dangerous creature gets hold of the minds of everyone present, turning them into his slaves. Meanwhile, at the same time as the creature is liberated, in Bellwood's convalescent home one of his tenants reacts and leaves, the elder is called by the nurse as "Old George".
Another faction of the Forever Knights is going to investigate the site of the events, but in the place Ben, Gwen and Kevin appear, both decrees make a truce to look for the creature and for the missing Forever Knights. After crossing the streets in search of the creature they find it in an alley possessing a policeman who attacks them. The Creature also possesses Gwen and Winston (the young squire), but it does not manage to possess Kevin since he was in his form of metal, Gwen manages to escape, whereas Winston is kidnapped by the Creature. All are released, though, after Ben as NRG makes a hole with the creature inside and Gwen creates another seal to trap it. It is revealed at the end of the episode that the Lucubra planted something in the people it possessed, when Winston's eyes turn green again.
At the Area 51 base, Old George appears and defeats all of the security guards. Seconds later, Area 51 disappears.
Old George is finally identified in The Purge, where it is discovered that Area 51 was turned into his new headquarters. He is recognized as the First Knight, a medieval warrior who founded the Forever Knights a thousand years ago. He unites the factions into one group and he assigns Driscoll as his second in command. After Driscoll lost a battle with Ben as Ultimate Spidermonkey, he goes to inform his failure to Sir George. He expects anger from The First Knight, but George is unfazed. He informs Driscoll that he is leaving on a quest, and that he will be in charge until then. He informs Driscoll to prepare the knights for when he returns, for "The Battle of a Hundred Lifetimes." And with that, he leaves.
It was a dark and stormy night at a carnival where there is a man running a freak show stand. A man is selling flyers for the group called the Flame Keepers Circle. He accidentally drops the papers, and when he picks them up, he looks into the tent and sees who he thinks to be Diagon, though he is actually Vilgax. Later, a group of Esoterica come, beat up the manager of the freak show, and liberate Vilgax. Later, Julie joins the Flame Keepers' Circle because she thinks that they will bring peace to the world, but Ben and Kevin are skeptical. Julie gets very mad at them and storms off. Later, Ben as ChamAlien sneaks into the building and checks out a door that he was curious about, and when he opens it, much to his surprise, is his arch-nemesis, Vilgax. Vilgax tells Ben that after his ship blew up, he washed up onto the shore and the circus man found him. He was regaining his health in the tank when the Esoterica found him and took him here. Now he is treated like a god. He tells Ben that he will return to his empire and Psyphon and then destroy the Earth. Ben reveals that Vilgax's empire fell because they thought he was dead. This annoys Vilgax so he calls his Esoterica to attack Ben, claiming he is an enemy of the Diagon. Ben attacks them as Big Chill and Ultimate Big Chill, but surrenders when Conduit Edwards threaten Julie. Ben reveals that this is Vilgax, not Diagon, and she gets away from Conduit just as Gwen and Kevin arrive. Ben shatters Vilgax's tank with Echo Echo and leaves. When the Esoterica are repairing the tank, one of them questions whether Ben was telling the truth about Vilgax. Vilgax replies that when he's done with the Earth: "You won't even recognize it".
Still impersonating Diagon, Vilgax demands the Flame Keepers' Circle to take him to Diagon's heart in order to regain his power, meanwhile, Ben, Gwen, Kevin, and a team of Plumbers have been sent on a mission to stop Vilgax from making a getaway. Ben and his gang were no match for Conduit Edwards and the Esoterica, but luckily they caught one of them after they escaped. When they unmasked one it is revealed to Sir Cyrus's squire Winston. With clueless looks on their faces, Ben, Kevin, and Gwen asked that how long Winston has been a member of the Esoterica, but somehow, Winston had no memory about how he winded up with them and why. It turns out, that ever since he was mind-controlled by the Lucubra before it was resealed, Winston was also used like a puppet by the Diagon to spy on Vilgax and find out his plan. After the gang meet up with Driscoll, Sir Cyrus, and the Forever Knights, Diagon suddenly invades Gwen's mind (because of her mental link with the Lucubra) to deliver a warning message to Ben and the others about Vilgax going after his heart. After Gwen gets freed from Diagon, Driscoll tells Ben's group the story of Old George and how he defeated Diagon (as a dragon from Driscoll's perspective), and what happened to Diagon's heart. Ben figured that George must be immortal, and Gwen helped searching for clues by solving George's calculus he wrote all over his quarters to find where the location that housed Diagon's heart is. Later, the knights reunited with George to point out an invisible fortress that appeared between both Earth, and Diagon's dimension. Suddenly, the Esoterica appeared and made an ambush on George and the knights while Vilgax reached the heart by touching a mysterious sword carved in it and thus, regaining his original form and equipped with knight armor. After possessing Winston again, Diagon tells Vilgax where to find more power by sending him back to the seal that the Lucubra broke out of. Ben and his team arrived at the same location and see Vilgax breaking the seal and George and the knights unconscious. The gang failed to beat Vilgax now that he's armed with full power and a sword, but after putting a crack to the seal Vilgax is being pulled into a blind light via sucked and trapped into Diagon's dimension, plus dropping the sword and leave it behind. Old George regained conscious, as Ben and the others awake, and reclaims the sword he calls Ascalon and suddenly regained his youth along with armor, telling Ben's team to "let the dragon come".
Much to their shock, Ben, Gwen and Kevin witnessed that George is the first knight who slayed Diagon over a thousand years ago. George tells them to bow but Kevin refuses. Suddenly, Azmuth appeared from nowhere and claims that the Ascalon belongs to him. Regardless of Ben's confusion, Azmuth demands George give his sword back to him, but he refuses by responding that "not while the Diagon lives", so Azmuth tells Ben to fight. Ben uses Fasttrack again but misses when George evades his speed attack, so the rest of the group start to join in, with Ben switching to Humungousaur next, Kevin absorbing stone, and Gwen powering up with her mana. The team were no match for George when he's become armed with his sword, by demonstrating a whirlwind with full speed at Humungousaur, and turn him back into Ben. After George retreats via a space-time rift, Ben forces Azmuth to tell the truth about the sword and how George gained it. Azmuth then shows the team a holographic flashback-like virtual reality sequence of a younger Azmuth on a picnic date with his long-forgotten lover Xennith on Primus, where sees an alignment of planets which inspires him to create the same sword Sir George possessed. Despite the consequences Xennith warns Azmuth about, Azmuth now sees the biggest mistake of his life when the power of Ascalon caused the destruction of the Incursean homeworld. Azmuth later tells Ben and the team that another reason he created the Omnitrix and Ultimatrix was also an apology to Xennith for what he built before. So Azmuth decided to give Ascalon to someone worthy to wield it without being overcome by the power, like George, who appeared to weary and exhausted after a battle with the Lucubras. George used the power of Ascalon to defeat Diagon by cutting out his heart and banish him back to his dimension by forging the seal. After the "flashback" ended, Ben, Kevin and Gwen head for Area 51, which became a base for all of the Forever Knight factions, for Ben to have another fight with George and try reason with him to give up the sword so he wouldn't underestimate the Diagon when he returns for a rematch. Ben calls it a truce by making a deal with George that if he loses the big battle, he'll hand Ascalon down to Ben. After the team returned to the cave, Azmuth hopes that Ben knows what he's doing while preparing for an ultimate war.
Two weeks later after Azmuth revealed the whole story about Ascalon, Sir George, and the Diagon, Ben, Kevin and Gwen are being ambushed by the Esoterica in Kevin's garage, which they learn that the Flame Keepers' Circle are about to free Diagon from his imprisoned dimension.
On the other hand, George leads his Forever Knight army to the location of The Seal against the Esoterica in an all-out war. Thanks to Ben's team's interference, they have to fight off the Knights inside the Rust Bucket 3 and the Esoterica out in the battlefield. While inside the cave, George and the rest of his Forever Knights were just about to reach the seal when Conduit Edwards appeared and launches a sneak attack on George, only for Winston to sacrifice himself to save his leader's life. George then fights off Conduit, and kills him with his sword in the process. Sadly, Winston dies after stating that it was an honor to be George's squire, but to George, Winston died a knight and his death won't be in vain.
Upon arriving at the Seal, George hears Diagon's voice when Diagon claims that he will be free enough to rule the entire Earth's dimension. So, Diagon summons his herald, which is revealed to be Vilgax, merged with a Lucubra, to destroy Sir George along with the other Forever Knights. Suddenly, Ben blasts in through the ceiling in the form of Heatblast and joins forces with George to stop Vilgax from breaking the Seal.
As the war between the Forever Knights and the Flame Keepers' Circle continues, Gwen and Kevin, now using a fallen green weapon after he upgraded with his Plumber's Badge, are taking out the Esoterica soldiers while Heatblast and Sir George are working together to battle Vilgax, who announces, "The ruination of this universe is at hand, The Diagon comes!" by destroying The Seal and thus freeing his "master".
Gwen and Kevin arrived in the cave to aid Ben and George, but they were no match for Vilgax. Thanks to him, the Seal was damaged enough for Diagon to send a wave of energy to transform every living human on Earth into an Esoterica, except for the team when they were luckily protected by Gwen's mana shield, and Julie, saved by Ship in armor form. Ben tells Gwen and Kevin to go home at Gwen's house to find a containment spell, while he, switching to Eatle, and George continue fighting Vilgax, only this time in Diagon's home Dimension. After arriving at Gwen's home in Bellwood, she and Kevin fend off more Esoterica soldiers while going through every page of Gwen's spellbooks to find a spell that would recontain Diagon, but found nothing. Suddenly, Diagon invades Gwen's mind and takes control of her entirely and uses her and her powers to return to the Seal's location at the speed of light. Fortunately, Kevin took an abandoned car to follow her. Back in the cave, Diagon was angry because Vilgax wasn't using enough power to break The Seal. Ben, now changed into Humungousaur, and Sir George still attack Vilgax, but he managed to knock them both out of Diagon's realm and back into the cave. Kevin arrives and tells Ben that Diagon's got Gwen. Due to Vilgax's failure, Diagon now makes Gwen go Anodite, approach the Seal, and destroy it for good. Humungousaur approaches her, and tries to stop her, but Gwen uses her powers to knock him out, and forces him to transform back to human. Then, Kevin runs up to her and grabs her arms, then covers her in the metal coating he was wearing, before pulling it back onto himself, with the exception of a metal cap over her head, to prevent Diagon's control. She thanks him and Kevin creates a metal cap around Ben's head too, as well as offering one to George, who refuses because he has the Ascalon to protect him from Diagon's mental control. Meanwhile, Psyphon appears in his ship and exits it to meet Vilgax, and claims that he has gathered enough equipment Vilgax demanded. Ben changes into Jury Rigg, as he and his team readied themselves for a final battle against both Vilgax and Psyphon.
Suddenly, the Seal has been completely shattered, and both the team and Vilgax and Psyphon ran outside and looked up in the sky in shock and horror to find that Diagon is finally free.
The Diagon appears in the sky, announcing Ben, George and the others to "behold his might". George makes his first move, striking Diagon across the face from distance with his Ascalon, thus leaving a couple scratches on the Diagon's face and hurting him, until he heals them instantly. Diagon commands Vilgax to destroy George, but somehow Vilgax disappeared, and Psyphon along with him. Instead, Diagon summons his Esoterica minions to attack George, only until Kevin and Gwen help by fighting off the Esoterica, keeping them occupied. Ben, on the other hand, switches from Jury Rigg to Way Big, and, for the first time ever, Ultimate Way Big and starts to fly upward towards Diagon and pulls him down into the river and destroying the dam in the process. Ultimate Way Big begins shooting cosmic rays and discs at his foe, however, none of his attacks cause any significant damage to Diagon. Annoyed by Ultimate Way Big's attempts to stop him, Diagon creates a small acid storm over Ultimate Way Big, harming him long enough to cause the Ultimatrix to time out, leaving an unconscious Ben in the water. Diagon continues his fight against George while Gwen and Kevin still taking down multiples of Esoterica. Ben regains consciousness and returns to the battle field as Ultimate Wildmutt and quickly defeats the Esoterica, then he changes into Chromastone to help George by firing continuous energy attacks at Diagon, until Diagon knocks Chromastone out, turning him back to Ben, and zaps George with a bolt of red lightning, resulting George to revert into his old man appearance, drop Ascalon, turn to stone, and be blown away as dust. Now that George is dead, Diagon will begin to conquer our dimension. Suddenly, Vilgax reappears, with Psyphon and a machine by his side, declaring that if anyone is to rule this universe, it's going to be him; he also plots to take over Diagon's universe as well. Diagon starts to attack Vilgax by destroying his "weapon" (which reveals to be the same machine Vilgax used to absorb powers from Ultimos and other heroes from 9 worlds he conquered). However, his laser beams are absorbed into the machine as it keeps drawing in his power. Psyphon explains that under normal conditions, the device simply drains a being's powers. However, since Diagon's very being is made of power, he gets absorbed by the machine. Upon completely absorbing Diagon, Vilgax demands his power be given to him. Ben tries reasoning with Psyphon, but he doesn't listen. Upon gaining Diagon's powers, Vilgax takes a form similar to him and shoots laser beams at the team, leaving nothing but a smoldering crater where they were.
Fortunately, Gwen teleported herself and the team to The Plumbers' base at Mount Rushmore in the nick of time when Vilgax's lasers almost hit them, but that didn't stop Vilgax as he reappears again a second later and commands the Esoterica to attack the base until Gwen activates the base's defense system, warding off the mindless drones. Annoyed, Vilgax resumes his previous form and rips off the Washington face from the monument, disabling the defense system. Once inside the mount Rushmore fort, Vilgax faces Ben in a final stand-off, asking what next transformation he would use, only to hear Ben responding that he's not gonna use the Ultimatrix this time and see him being covered in a knight's armor from equipping with George's sword. Vilgax fires energy blasts at Ben, but he reflects them back at Vilgax with Ascalon. They both charge at each other, and in a flash of light, Vilgax is reverted to his normal form; Ben absorbed Diagon's power into Ascalon. When Vilgax questions what he'll do with Diagon's power, Ben tells him that he'll change the Esoterica back to normal. However, Vilgax says that with the Ascalon, the Ultimatrix and Diagon's power, he could rid the universe of evil for good. Ben finds this interesting, but Gwen and Kevin, recovered from a knockout by Vilgax, ask if he should take moral advice from an enemy, like Vilgax. Then suddenly, Julie and Ship arrived, telling Ben that they and others Ben knows love him, that if he goes through with this, he'll be as evil as Vilgax, Diagon, or Aggregor. Gwen exclaims to Ben that he's tempted with power just like herself as a full Anodite, and Kevin when he was Ultimate. Ben, pressurised with his decision, demands for silence as he thinks of all his adventures, from the day he first met Julie to his travel to space to rescue both Ship and Baz-L. Then, Ben raises Ascalon and sends beams of energy to turn every Esoterica on Earth back to human, just as he stated before. Ben's armor magically removes, and Azmuth suddenly appears to get his sword back, so Ben returns it to him along with the Ultimatrix. After being proved worthy, Ben giftedly earns a new Omnitrix from Azmuth, who explain that it is the Omnitrix, and that the original and Ultimatrix were both inferior prototypes.
Ben thanks Azmuth, and joins together with Gwen, Julie, Ship and Kevin to live peacefully and happily ever after.