[Barney enters MacLaren's dressed like an elderly man, complete with prosthetic make-up]
Barney: You there, what's your name?
Cindy: Excuse me?
Barney: Your name, woman, what's your name?!
Barney: I knew it. You're the Cindy, the one who 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.
Cindy: The future?
Barney: The future, and I can prove it to you. In exactly four seconds, the woman at that booth is going to slap that man. [Barney clears throat]
[Robin slaps Ted]
Ted: Ow! What the hell?
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.
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.
Cindy: 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!