Barney and Robin have gotten so comfortable in their relationship that Barney has put on a huge gut, and Robin looks like a makeup-less zombie that just hates everything he says. Ted and Marshall realize how unhappy these two are, but they're engaged in relationship chicken... neither will end it. They attempt to break the two up by sneaking a engagement ring into Robin's champagne glass thiking it will scare her off, but neither will back down in the game... they agree to get married. It's clear Ted and Marshall need someone more powerful to help... the Kraken (Lily).
Lily hatches an intricate plan to bring up all four of their major fights: Star Wars, dirty dishes, the Canadian-American war, and Barney's crazy ex-girlfriends. The plan goes horribly wrong but it doesn't matter, Barney and Robin already broke up before it was executed. Turns out it was a rough patch, the relationship just ran its course. At the end of the episode, Robin is back to looking beautiful and Barney is skinny, single and ready to mingle again.
Yay for having our old dynamic back.