Ben 10 season 2 episode 4 : Gwen 10

This episode is a "What If?" scenario that is separate from the main story. When Ben wakes up one morning, he finds the Omnitrix missing from his arm. Weirder still is the fact that Gwen and Grandpa Max have no idea what he's talking about when he asks about it. Ben soon discovers that he's actually reliving the very first day of summer, the same day that Ben found the Omnitrix to begin with, but in an alternate reality. At the campsite, Ben tries to find the Omnitrix as it crashes to Earth, only to find out that it has already been found by Gwen. Gwen quickly adjusts to the Omnitrix's powers, with much greater skill than Ben. However, unlike the first episode Vilgax was not injured at all earlier and is already heading for Earth to take it back from her, preferably in a very painful way. Ben's going to have to prove himself to be a hero without the Omnitrix to back him up if he wants to save Gwen.