When Robin started getting annoyed about Barney being an insensitive boyfriend and complained in front of Lily, Ted overheard and warned Barney. For him to practice teaching and for Barney to learn how to be a better boyfriend, Ted began teaching him a Robin 101 class. He taught Barney such important facts as how to read her face, calm her down, and even her bizarre erogenous zone -- her left knee.
Barney starts acting like a better boyfriend, buying her flowers, but when she notices there's certain hours of the day he's disappearing (while in class), she suspects he's cheating on her. She breaks into his briefcase and finds his notebook from class and Marshall reveals what's been going on. Robin crashes their class and is infuriated at Barney for cheating by trying to skip the steps of getting to know a person.. plus half the stuff is wrong! Or at least the knee being an erogenous zone.
However, thanks to Ted telling Robin how hard Barney's trying to keep her and then a similar confession by Barney, the two live happily ever after... until next epiosde.
Meanwhile, the B-side story was Marshall cleaning out the basement storage of their old apartment and finding an old beer barrel he once used as a nightstand. He goes to give it away by putting it in the "Bermuda triangle" -- a spot in front of their apartment where whatever they put out disappears in minutes. However, unlike Barney's notes which are left outside for a few minutes and swiped, nobody take the barrel.