Gwen steals a book from Hex that allows her to travel back in time so she can stop the Omnitrix from overloading. Professor Paradox intervenes just before the spell is activated, warning Gwen that the consequences for "fooling" with time are extremely severe. Gwen activates the spell anyway. She convinces her past self to stop Ben and Kevin from starting the machine that overloaded the Omnitrix. Her plan succeeds and she returns to the present where she finds a group of humans being held hostage by Charmcaster's minions. She attempts to fight them off, but is outnumbered and runs into Paradox, who explains that her interference has created an alternate timeline. He tells Gwen that Kevin wasn't able to help fight off Hex and Charmcaster's forces because he didn't have his monstrous form's abilities and was turned into a monster minion. The alternate Gwen, however, was killed by Charmcaster. The current Gwen breaks into Hex's building and after fighting Charmcaster, she escaped by cutting the ground around her and falling through all the other floors into a torture room. There she finds an injured Ben.
They fight Hex, Charmcaster, Kevin and her other minions and after Ben buys Gwen time, Gwen activates the time spell again. This time, she convinces both her other past selves not to interfere and wipes the memory of the Gwens that belong in that time period. The two time traveler Gwens go home and the present Gwen takes the present Ben, and present, mutated Kevin to Mr. Smoothy's. On the way, the gang runs into Paradox again who says everything is fine.
It's revealed that Hex and Charmcaster are to be very dangerous, as Kevin only being slightly weaker was the difference between victory and defeat.
Past Gwen 1: (conversation continues)...Are you sure? Past Gwen 2: You will never forgive yourself if you let this happen. Gwen (Original): Don't listen to her! I'm from her future and she doesn't know what she's talking about! Past Gwen 2: Oh come on! How do I know you're not an impostor? Gwen: Ugh, the lock on our locker is broken, we just leave it hanging there so it looks closed... Past Gwen 2: Alright, you're me...but I still think we should save Kevin. Past Gwen 1: You guys need to make up our mind! Gwen: Already done. Paradox was right! We have to let it happen the way it did the first time. Past Gwen 2: We can't keep Kevin from mutating? Gwen: No...we can't, (puts hand on Past Gwen 1's forehead and wipes memory) and just to make sure she doesn't try—I'll make her forget everything that happened... (turns her around and walks off with Past Gwen #2) Come on...(walks off) we don't belong here...
Paradox: There's an old saying in my field; Time travel is only for immortals and fools. And I speak from experience. Gwen: Professor Paradox? Paradox: Hello, Gwen. Gwen: What are you doing here? Paradox: I'm about to warn you not to go through with this plan of yours.
(Looks at his pocket watch) See, I just did. Right on schedule. Now comes the part where you tell me... Gwen: This spell can send me back in time! Back when Ben and Kevin tried to hack the Omnitrix! Paradox: Yes, and you think if you can stop them there won't be any explosion and Kevin won't be transformed into a monster. And everything will be wonderful again! Gwen: Yes! Paradox: No! Generally, 2 out of 3 isn't bad, but in this case however... Trust me, Gwendolyn. Don't do this! Gwen: But I haven't been able to cure Kevin with my magic. It's the only way. Paradox: There's always another way. In fact, sometimes to do something, all you need to do is nothing. Gwen: (starts crying) You're not making any sense. Paradox: I'm often accused of that, but I'm the time walker and I'm generally quite well informed on these matters. Gwen: I have to do this. Paradox: I suppose you do. But mark my words, Gwen Tennyson. You are about to unleash forces that you can neither comprehend nor control. Toodle-doo.
Gwen : (as she gets up from the ground and looks around,she sees Spidermonkey, who is moaning softly with pain, hanging from his wrists from the wall. She gasps) Ben! (she runs to him and uses her powers to break the chains holding Ben, who falls to the floor and switches back to his human form, his eyes closed. Gwen helps him sit up, as Ben opens his eyes) Ben: (weakly, in a faint voice) Gwen? But...I saw Charmcaster...you were fighting her and she... Gwen: I know, Ben...Its all my fault... Ben: (Still in pain) but I don't understand. Gwen: The Omnitrix... when it blew up... only it didn't... but it should have and would have if only I hadn't tried to fix everything. Ben: I know I'm kinda messed up, but you're not making any sense. Gwen: I'll explain as soon as I get you out of here. (They stand up and move two steps but Charmcaster breaks through the wall with a rock monster and Kevin) Charmcaster: Oh sure, like that's gonna happen.
Ben: (to Gwen) You handle the rock monsters, I'll take Charmcaster.(suddenly bends over, gasps in pain, and puts a hand to his chest, his eyes closed) Gwen : (holding on to Ben's arm) No, Ben, you're hurt. Ben : (standing upright again, but still looking as if he's in pain) I've already lost you once. Not gonna let that happen again.
Hex: It's time to write the last chapter in this sad little saga. Swampfire: Thanks anyway, Hex, but I'm not much of a reader! (lets loose a jet of fire which causes the ceiling above Hex to crumble and knocks Hex out...Gwen summons the book of spells) Swampfire: Let's get out of here, before they have a chance to- (the Omnitrix crackles with sparks as Swampfire falls to the floor in a curled position, and Ben turns back into human and moans in pain as Gwen kneels beside him, her hand on his back)
Charmcaster: Gwen Tennyson? No way! Gwen: Why so surprised? Charmcaster: Cause I killed you once. And now, I'm gonna do it again!
Gwen: And Ben? Paradox: He fought bravely, but eventually, they caught him too. Pity. After you died, he was the planet's last hope. Gwen: No! Dead?! Me?! Paradox: Yes! Dead! You! Why are we talking this way?
Paradox: (to Gwen) I warned you that time travel is for immortals and fools. I'm the only immortal hereabouts. You do the math.
Gwen: (after fighting Charmcaster and sending her to the ground) Killing me is not cool! (hits Charmcaster with her mana again)