Lily is sadder to learn that Barney and Robin aren't a couple more so than Barney or Robin. But after that, Lily catches Barney and Robin making out, the two who confess that they have been in a sexual relationship over the course of the summer, a fact they were hiding since they didn't want to make a big deal out of their undefined relationship. Lily thinks that Barney and Robin need to have "the talk" to define what they really mean to each other. They refuse since they are quite happy having sex and that's it. When Robin is forced to go on a date with Brad, Barney and Robin almost get to the point of feeling the need to have the talk. When they still can't go through with it, Lily, ultimately with some help from Marshall, comes up with a plan to force them to have the talk. Even if they do have the talk, will they come to any conclusions? Meanwhile Ted is preparing for his first day as a college professor teaching Architecture 101. He is trying to decide what kind of professor he ..