Plot

Barney is upset because he violated his own "bro code" by sleeping with Robin. How will Ted react when he learns that Barney slept with his ex-girlfriend?

As he started comforting her following Simon dumping her for the second time in her life, Barney ended up sleeping with Robin. In the aftermath, both Robin and Barney want to pretend that it never happened. It ends up being seemingly easier for one than the other. Barney feels that his actions are making him looking guilty especially in front of Ted, to who he feels he broke his self-written bro code in sleeping with the ex of a friend. As such, Barney hires an unemployed Marshall to be his lawyer. Marshall's legal task is to go through the bro code to find a loophole that made it OK for Barney to sleep with Robin. Meanwhile, Ted's thirtieth birthday is approaching, for which the gang are throwing him a not so surprise rooftop birthday party. The party may be ruined by Barney and Robin's secret, and by Lily's new goat, which she rescued from slaughter. As Ted relates this story to his children in 2030, he admits upon completion of the story that he got a large portion of it incorrect.—Huggo

Its Ted's 30th birthday. The episode begins with the aftermath of the events of "Sandcastles in the Sand": Barney and Robin are in bed together after having sex and she proposes to pretend it never happened.

Barney agrees but feels quite awkward and uncomfortable around Ted at MacLaren's later in the day. Every conversation at the table makes Barney feel that the gang is talking about him having had sex with Robin the night before. When Ted offers Barney his Xbox, Barney is offended and says that she has a name. Meanwhile Marshall has told Ted that the gang is planning a surprise birthday party for him. Ted asks Barney to invite Stella.

Hoping to find an excuse for his and Robin's actions, Barney seeks out Marshall to help him (as a lawyer) find a loophole in the Bro Code, a book listing the rules and philosophies of Barney's life as a "bro".

Allegedly written by Barnabas Stinson in the 18th century when Benjamin Franklin and George Washington were too busy to write it themselves (They wanted a set of rules to define acceptable behavior between friends when it came to girls), the Bro Code proves to be a very tight document (Bros before hos, The mom of a bro is off limits but the step mom is fair game if she initiates it or is wearing at least one article of leopard print clothing, Bros cannot make eye contact during a devil's three way (2 dudes)) that Ted has followed flawlessly (a bro at all times shall say yes (Ted said yes when Barney claims to a girl that he saved him from an avalanche, carried him 6 miles to safety on a broken leg), a bro will in a timely manner alert in case of a girl-fight, A bro will whenever possible provide his bro with protection) and Barney fails in finding a loophole. The bro code clearly states no sex with a bro's ex. Marshall says Barney feels bad as he knows what he did was wrong. He tells Barney to come clean to Ted.

When Marshall's nervous behavior reveals to Robin that Barney told him, she tells him that Ted is never finding out. That night, when Barney picks up Ted in a limo (to take him to a surprise birthday party that Lily has arranged for him on the apartment roof), he attempts to give Ted an ultimate 30th birthday by flying him to Vegas, which includes staying at the Bellagio, steaks at Boa, and a boxing match between Floyd Mayweather and a grizzly bear. Ted reveals that he knows Barney and Robin slept together, for Robin told him right after she warned Marshall. Ted, at first, seems calm, and Barney is relieved, but Ted screams at him, angrily declares they're no longer friends (after Ted hits Barney in the private parts when Barney offers him one free shot anywhere on his body but the face) (Ted says that he had seen Barney do a lot of bad stuff to many people, but he always felt Ted was the limit and that Barney would never betray him), and hails a cab to take him home to his friends. Barney then sadly sits in the limo alone all night.

Meanwhile, Lily rescues a goat named Missy when a farmer (Farmer Frank, (Alan Fudge)) brings it to her kindergarten class and horrifies her students by going into great detail about what will happen to Missy when she visits the butcher. Lily plans to give the goat away to animal control (who will only come to the apartment after the weekend on Monday), but becomes attached to the goat while waiting for Ted to arrive at his birthday party. Older Ted reveals at the end of the episode, after hinting at the destruction the goat would do, that he had the story wrong: Everything involving the goat actually takes place during his 31st birthday, at which point - Older Ted also reveals - Robin is living in the apartment.