Recap
The episode begins by Ted walking in on Robin's date, Mitch, sitting naked on his couch. We learn that when Mitch realized his date with Robin was going nowhere, he was going to use the "Naked Man." The Naked Man is a technique where you find an excuse to get to the girl's apartment and strip down naked. When she walks in on you she's so shocked, she'll find it funny and have sex with you. It works 2 out of 3 times. Guaranteed.
After finding out the Naked Man worked on Robin, Marshall calls Robin a slut and thinks the only thing standing between her and sex is "clothes." Robin is desperate to prove she's not a slut and goes on a second, terrible date with Mitch. Meanwhile, Lily starts a list of 50 reasons to have sex to prove to Marshall there's more reasons than just love. Ted tries to take his flirting with Vickie, the cute girl from the elevator at work, to the next level and Barney is determined to use the Naked Man on a woman that night.
Ted's date with Vickie goes terrible when he finds out she's an awful human being and decides to try the Naked Man. Barney manages to sneak his way into a girl's apartment to see her aquarium and tries the Naked Man. Lily, to prove Marshall wrong on sex is only for love, tries her own Naked Man. Since the move is only guaranteed to work two out of three times, one of our heroes had to fail. It was hilariously Barney.