Ben 10: Alien Force, Season 3

Professor Paradox:
There's an old saying. Time travel is only for immortals and fools and I speak from experience.

Gwen:
Professor Paradox?

Professor Paradox:
Hello Gwen.

Gwen:
What are you doing here?

Professor 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!

Professor 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 wonderfull again!

Gwen:
Yes!

Professor Paradox:
No! Generally, 2 out of 3 isn't bad, but in this case... 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.

Professor 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.

Professor 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.

Professor 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-oo. (vanishes)