Plot

Ted meets with his ex-girlfriend from college--a girlfriend who cheated on him constantly and Marshall and Lily can't stand, and Marshall tells the story about how he forgot to bring his pants to work.

While Ted, Barney, Marshall and Lily have a beer at the bar at 2am, Robin, who drops by the bar, has just woken up to start her new job at the morning show. Barney is relaying a story about Marshall at work that day, he who was getting dressed for work after a basketball game at the company gym when he realized that he forgot to bring his suit pants, meaning that he had to go to work wearing his athletic shorts. Barney plays a hand at what Marshall ultimately wore for the rest of the work day. And Ted is telling the gang that his old girlfriend Karen is in town. Karen was Ted's first real girlfriend in high school and through some of college. She was pretentious and chronically cheated on Ted, which are the primary reasons that Marshall and Lily hate her. But Ted always took her back when she came running. As Marshall and Lily try to persuade Ted not to fall into the same trap he always has with Karen let alone even see her, Ted fills in a few more of the blanks regarding what he has already done with her on this visit.—Huggo

At 1:45 am, Robin gets up so she will be in time for her 4 am talk show that she now anchors. She often stopped at the bar (where the gang was still drunk from the previous evening), before she headed out to her work. Meanwhile, Ted tells his friends that Karen (Laura Prepon), his girlfriend during high school and college, is now in town.

Marshall and Lily are not happy with this news. They tell Ted how annoyed they were by Karen's pretentiousness (she claims people living in dorms only eat baloney sandwiches and are racists) and how Ted imitated her opinions (she didn't like TV, and once pretended to have bought tickets for wrestle mania making Marshall and Lily dress up in the wrestling costumes to visit the show), and how Karen always cheated on Ted, left for a few months (during Marshall & Lily told Ted how much they hated her), and then got back together with him (after he goes to Karen to give her a piece of his mind). And then the same thing will start all over again.

The gang agrees he definitely should not call Karen (it would be the 4th worst thing in the world after: super volcano, asteroid strike, all footage of evil Knievel is lost, Ted calls Karen, Lily gets eaten by a shark). Ted admits he already called her. They say he should not ask her to lunch, but Ted admits he already did. The group discusses the four different reasons one would have lunch with an ex: They want to get back together, as depicted in Lily's lunch with Scooter; they want to kill you, depicted in Barney's lunch with Wendy (when in fact she only wanted to give him back his tie); they want to give back something you left behind, depicted in Robin's lunch with Curt "The Iron Man" (who returns Robin's .38 snub nose police special to her); or they want to show off how well they are doing, depicted in an eight-year-old Marshall having lunch with eight-year-old Nicole.

Ted reveals that he has entered a relationship with Karen (he kissed her after lunch & then brought her back to his place for sex), facilitated by the strong sleeping pills Robin has been using to help her keep up with her schedule (due to which she never realized that Ted was bringing Karen back to their apartment to have sex with her at nights). Lily is extremely frustrated to hear this, and states the true reason she hates Karen: while she was painting Marshall in the nude, Karen entered and "lingered".

Ted tells the rest of the story: while they were hooking up in her apartment, Karen's boyfriend entered. Apparently, this time Karen was cheating on her boyfriend Jerry (Houston Rhines), with Ted. Ted says "Sorry bro", which was something that the boyfriend had said to him numerous times over the years. After the boyfriend left, Ted confronted Karen about it and said he never wanted to speak to her again. Karen later dumped the boyfriend. The gang is happy to hear this, but asks Ted how he could know Karen dumped him if he never spoke to her again. Ted quickly says that he and Karen called each other again, patched things up, are now back together and that she is coming to the bar. Karen enters the bar and the gang unhappily greets her.

The previous day at work, Marshall was playing basketball with his office colleagues and forgot his suit pants. He called Lily to come over and give him his pants. Lily gave the pants to Barney to give to Marshall (as she was in a hurry), but Barney cut them up. Marshall has an important meeting and he had to show up wearing nothing but pants that had been cut up into shorts by now. Barney finds this story amusing and tries to tell it throughout the evening, while Marshall and Lily are much less happy about it.