Marshall: I guess it's 'cause we never hang out alone together.
Robin: Why is that?
Marshall: It's 'cause of The Mermaid Theory.
[flashback to Marshall at work:]
Marshall: [v.o.] Two years ago, I had just hired a new assistant at GNB.
Barney: So, who's the eye broccoli?
Marshall: Hey, that's Iris, my new assistant. And yes, she's a little plain, but I'm married, so that's good.
Barney: Mark my words, Marshall. Someday you will find Iris so excruciatingly attractive, you won't be able to look her directly in the boobs.
Marshall: I don't think that's gonna be a problem.
Barney: Marshall, do you know how the myth of mermaids came to be?
Marshall: I'm sorry. Myth?