Ben 10 season 3 episode 3 : A Change of Face

While Ben and family are visiting Salem, Massachusetts, Charmcaster attacks in an attempt to switch bodies with Ben; however, Gwen interferes, causing Charmcaster to switch with her instead. Gwen gets arrested in Charmcaster's body while Charmcaster stays with the unaware Ben and Max and tries to cook up another body-transfer spell. Gwen escapes from jail and returns, convincing Ben of the truth before Charmcaster tries again. However, Charmcaster manages to swap their bodies once more, ending with Charmcaster in her true body while Gwen and Ben have switched. The two swap back as a result of Charmcaster's third body-swap spell and Cannonbolt defeats Charmcaster. In the aftermath of the battle, Gwen ends up taking Charmcaster's spellbook for her own use and in hopes of building up her magic.