How I Met Your Mother, Season 4

Barney:
[in old man make-up] You there, what's your name?

Woman:
Cindy.

Barney:
Cindy. I knew it! You're the Cindy, the one that can change everything...or spell our inevitable doom. Now listen to me, Cindy. I am Barney Stinson. And I am on an urgent mission from the future.

Woman:
The future?

Barney:
The future! And I can prove it! In exactly four seconds, the woman at that booth is going to slap that man. [Clears throat loudly]

[Robin slaps Ted; woman is amazed]

Barney:
In a few minutes, the young me from your time is going to come through that door. Now, Cindy, I know this sounds insane...but in order to save the planet, you need to sleep with him. Tonight.

Woman:
What?

Barney:
Sleep with Barney Stinson, tonight, in whatever way he wants it...or he won't be able to find the solution to global warming that saves the human race!

Woman:
What are you talking about?

Barney:
I have no time to explain. I have to get back to the reality accelerator before the vortex closes. Only you can save us, Cindy. I must away!

[Barney returns a few minutes later, looking like himself]

Woman:
Oh my God! You're-oh my God! Can I buy you a drink?

Barney:
Well, I guess I have time for one drink, and forty-five minutes to an hour of some other activity. But after that I have to get back to, uh, a secret research project I'm working on.

Woman:
Global warming?

Barney:
My god. How did you know that?