While hanging out at Fort Knox, Gwen grows jealous of Ben's aliens getting all the glamour and attention. But when she discovers that a new villainess, Charmcaster, is really Michael Morningstar's last remaining fangirl turned witch, Gwen must withstand attacks from both her and Morningstar, and prove herself as a valued member of Team Tennyson.
The episode starts off with Ben and Gwen walking through Biggie Box, surprised that Max allowed them to go. A little kid walks up and recognizes Ben, pointing out he saved Adrenaland. The father picks him up, and thanks Ben. He also questions if Gwen was a fan, however Gwen corrects him, stating that she helped save the park too. However, he ignores it. Ben notices Gwen is sad, and he attempts to cheer her up, however, he only makes her more angry. As Gwen walks from the ice cream stand, an unknown figure is seen watching Gwen. A piece of paper is dropped on Gwen’s ice cream, turning it into a monster. Gwen tries to get everyone out of there, and she goes to hide. The monster continues to chase Gwen around. Ben transforms into Heatblast to fight it. He questions where it came from, and Gwen is confused, as she assumed it was Ben’s fault. A girl calls them from a distance, and it is revealed to be Charmcaster. She states that she is here to get payback for what Gwen did. Ben tries to introduce himself, however Charmcaster doesn’t put up with him, and sends him through a portal elsewhere. Heatblast continues to fight the monster; he manages to blow it up.
Charmcaster checks Gwen out, confused as to how a girl who appears to be no threat broke Michael Morningstar’s heart. She explains her origin story, and how she used to be weak until she one day found a book that taught her how to be powerful. It gave her the courage to visit one of her favorite movie sets, and there she saw Gwen and Ben fighting Michael. Charmcaster prepares to finish off Gwen until Michael shows up and stops her. He then drains a bit of her power to fuel his own. Ben arrives, and asks Gwen what’s going on. She explains that Michael has some sort of a hold on Charmcaster. Ben transforms into Omni-Enhanced Four Arms and attempts to attack Michael. However, Charmcaster defends Michael, and handles OE Four Arms herself. Michael chases Gwen around, angry that she once abandoned him.
Four Arms tries to ask Charmcaster why she’s with Michael, however it only makes her more angry. Meanwhile, Michael tries to absorb more of Gwen’s life energy, however he is unable to for unknown reasons. Gwen questions why he is doing this to Charmcaster, even though she loves him. He responds saying that all of his fans love him because he’s an actor, and he can be whatever they need him to be to fulfill their “pathetic lives”. He also says that she is nothing more than a glorified battery to him. Charmcaster, who manages to overpower Four Arms, arrives, questioning why Michael is still wasting time with Gwen, who she believes to be a “nobody”. Michael yells at Charmcaster, telling her they won't leave until he gets Gwen to see how “great he is”. He prepares to drain more of her power, however Gwen sticks up for her. Charmcaster questions why Gwen helped her, and Gwen explains that it’s the right thing to do, and that Michael is using her. OE Four Arms arrices and arracts Michael. Charmcaster runs over to Michael, and Michael absorbs more of her power. Four Arms is confused on how they’re going to beat him. Fortunately, Gwen gets an idea on how to beat him. Four Arms distracts Michael, who has now absorbed a large portion of Charmcaster’s energy. Michael is almost unbeatable at this point. Michael continues to beg for more power from Charmcaster. Finally, Gwen shows up with a remote to all of the TVs around them. Michael’s talk with Gwen earlier on was recorded, and Charmcaster finally sees Michael for who he really is. Now angry, Charmcaster sucks Michael into her spell book, trapping him in there.
Charmcaster makes it clear that though Gwen saved her today, that does not make them friends. On the road, Ben explains the entire story to Max. Max seems proud at the fact that Gwen was a full on hero today. Ben cheers her on, asking her how it feels to have her own arch-enemy. Gwen celebrates the fact that she has her own enemy. But then she pauses at a moment of realization.
"Charm Offensive" means a campaign of flattery, friendliness, and cajolement designed to achieve the support or agreement of others. Thus, the title of the episode is a play on Charmcaster's name as well as a hint at the plot.
The title for this episode in German roughly translates to 'Gwen's Archenemy'.
This episode aired as the 22nd episode of Season 2 and the 62nd episode overall in the U.S.
This is the Tennyson trio's second time visiting Fort Knox.