"Mosbius Designs" kicks off with a crude joke that begins: "What's the difference between peanut butter and jam?" The guys giggle about it at the bar, but when Barney whispers to Lily the punchline, she freaks out and leaves the bar. She's so offended, the gang doesn't see her for four weeks.
Back at the apartment, Robin has agreed to let Ted set up shop. Mosbius Designs seems to be up and running - Ted has a Web site, flyers, business cards, corporate events, an intern - and is even testing out official pens to write with. The only problem is he doesn't have any clients, and no amount of nagging on Robin's part seems to make a difference. He spends his days roaming the streets of New York City on so-called "Inspiration Walks" and giving his intern inane assignments, which the intern completes happily. That is until one day the intern calls Ted to say he'll be late for work he's on top of the Empire State building - but Ted's called ID reveals the intern is calling from inside the apartment. A quick check in Robin's bedroom reveals the intern is not on top of the Empire State Building after all...
Robin started sleeping with the intern because she's home all day and, well, he's always around. She admits that her standards for men are dropping. Ted, however, isn't happy with the situation since the intern's attention is now divided, so he fires him. The intern gets upset and calls Robin, and agrees to wait for her at her place - AKA Ted's office. But now that the intern is unemployed, Robin loses interest and dumps him. Then Ted hires him back, only to find that Robin is sleeping with him again, then fires him, which leads to another breakup. This vicious cycle drives Barney crazy since he is in love with Robin.
Since Lily is MIA, Barney gives up and decides to talk to Marshall about his Robin problems. He builds up to his admission of love, hoping for a big reveal, but it turns out Lily already told Marshall. The only one who doesn't know Barney loves Robin is Robin.
Marshall wants to help Barney out, but he is too focused on not losing his job to do much. There are rumors of more layoffs coming at GNB, and Barney tells Marshall that working hard has nothing to do with keeping a job. What Marshall needs is a schtick - the company has a "toy guy," a "food guy," and even a "shoulder massage" guy. All these "guys" are expendable, but their "guy" status makes them vital to the operation. And so, Marshall decides to start a fantasy baseball league and christens himself "Sports Guy." His new work identity isn't all fun and games though - somehow he ends up carrying $18,000 in cash, which he transports in his suit pockets, and becomes incredibly stressed out. He can't balance his fantasy baseball work with his real work.
But Marshall comes up with an ingenious plan that will fix things for every body. Well, everyone except Lily that is. In the end, Marshall solves Barney's problem and Ted's by hiring the Mosbius intern to take over his "sports guy" responsibilities. Robin stops sleeping with the intern just because he's there. Barney goes back to pining away for Robin. Marshall gets to keep his job AND his job as "sports guy," albeit with a little help. Ted even overcomes his fear of failure and decides to call a client.