The episode begins with Gwen, who dressed as Lucky Girl, sneaks into Hex's library and removes the Archamada Book of Spells from the shelf. Then, Hex enters into the library, but he does not sees anything. Charmcaster enters as well and assures him that there is nothing there. Gwen escapes with the book.
Ben and Kevin want to get into Gwen's house to watch football, but she isn't inside of the house. Kevin accidentally breaks the doorbell and Gwen arrives and lets them in, although she appears to distracted. Kevin thinks that her dismissal means that she doesn't like him anymore. In her room, Gwen uses the book to cast a spell, but she is interrupted by Professor Paradox, who cautions her not to cast her spell, which is a time spell that she intends to use to fix Kevin. Paradox tells Gwen that her meddling with the past will not have a good outcome, and that she doesn't have to do anything. Gwen doesn't listen and Paradox leaves and She casts the spell, which opens up a time vortex that sucks her in. Gwen emerges in the garage when Ben and Kevin try to hack the Omnitrix with a machine. Time stops for everyone in the garage except for Gwen and her past self. Gwen tells her past self to prevent Ben and Kevin from hacking the Omnitrix and she manages to do so. She returns to the present to discover Bellwood in a state of ruin, with a recording of Hex telling citizens to surrender and Stone Creatures that are everywhere. Two Stone Creatures then attack Gwen.Gwen defeats the Stone Creatures, only for her to be chased by three more. Gwen evades the stone creatures, but she bumps into another one. Gwen destroys the stone creature and escapes. While exploring, Gwen encounters another stone creature, but she avoids it. Gwen then goes to Mr. Smoothy and tries to call Ben, but he doesn't answer his phone. Gwen saves a group of boys from a stone creature and they run away. Gwen then goes to Kevin's garage and finds his car destroyed. A swarm of monsters attack Gwen there and she runs. Gwen sees a city street covered in Hex propaganda and Paradox arrives, telling Gwen that Kevin was too weak to stop Hex, so he took over the world. He brings her to Hex's fortress and tells her that Charmcaster enslaved Kevin and transformed him into a stone creature and says that Ben was captured, and that she was killed. Gwen watches Charmcaster abuse Kevin in horror and vows to save him. Paradox tells her that stealing the book won't be as easy this time, but she runs off anyway. Gwen is attacked by Charmcaster, who is surprised to see her.Gwen manages to block Charmcaster's attack and knocks her down. She enters the library and Hex appears, having expected her arrival. Charmcaster comes in and wants to fight Gwen. Hex tells her to be patient. Gwen attacks her and she summons Kevin to fight Gwen. Kevin attacks Gwen and she cuts through the floor. On the bottom floor, a dungeon, Gwen sees that Spidermonkey is tied up. Gwen frees Spidermonkey and he transforms back into Ben. Ben is surprised to see Gwen due to the fact that he believed that she was dead. Then, Charmcaster, a stone creature, and Kevin arrive into the dungeon and Ben wants to fight Charmcaster and he transforms into Swampfire. Swampfire and Charmcaster fight and Kevin attacks Gwen and she defeats him. Hex arrives and Swampfire drops the ceiling on him. Gwen takes the book and Swampfire transform into Ben. Gwen tells Ben to hold off Hex and Charmcaster while she casts a spell. Gwen runs away with the book and casts the time spell before Hex can stop her. Gwen stops herself from making her preventing Kevin and Ben from hacking the Omnitrix and she erases her younger self's memory and leaves. Back in the present, everything is normal. The team heads to Mr. Smoothy and Paradox assures Gwen that 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)
Despite his statement about karma at the end of Trade-Off, Kevin still angsts over his deformity.