Ben 10: Alien Force
Time Heals
S3 E15:
Despite being warned by Professor Paradox, Gwen travels back in time to prevent the accident that turned Kevin into a monster from ever happening. Gwen returns to the present only to find that her time traveling caused a series of events that led to Hex ruling the world. It's up to her to fix the future that she created.
Starring
David McCallum