Don

Synopsis
The park is about to be audited, with all of its property to be confiscated if the taxes are not paid. The park employees call Rigby’s brother Don, a friendly accountant, to manage the taxes. Rigby is jealous of Don, whose likable personality takes the attention away from him. Rigby turns away Don, who is saddened and leaves. Rigby then tries to manage the audit himself, but he (having no knowledge of accountancy) does it incorrectly. The audit begins and the park starts to get deleted like a computer program. It's up to Mordecai and Rigby to try and bring Don back to stop the audit.

Plot
Mordecai and Rigby are playing football with laundry when Benson yells at them to go outside for an emergency meeting. Benson says that the park is being audited because Pops tried to pay the park's taxes with lollipops, considered counterfeit, unknowingly to Pops. After telling Muscle Man to leave for not taking the situation seriously, Mordecai offers a suggestion, Rigby's accountant brother, Don. Rigby dislikes Don because Don stole all the attention from Rigby on Rigby's sixth birthday party. Despite Rigby's objections, Mordecai brings Don to the park, where he immediately takes the attention from everybody because he is cool, nice, funny, and the total opposite of Rigby, and also because he gives a hug/hand-shake thing he calls 'sugar'.

Rigby's hatred for Don however grows, and it goes far enough that he mocks Don and throws a football in Mordecai's stomach when he was hanging out with Don. Eventually, when Don is almost finished with the taxes, Rigby drives Don away after saying that Don should never have been born. Mordecai finds out and tells Rigby to bring Don back, but Rigby refuses to and says he can finish the audit by himself. However, he does it wrongly, and the park begins to disappear by the audit. Benson and the others try to get Rigby to bring Don back, but Rigby's frustration is too big. Mordecai tells Rigby that although he doesn't have a brother, he believes Rigby is his brother, despite how annoying and immature he can be.

This convinces Rigby to go with Mordecai to Don's house in order to bring Don back, but Don is too sad to go back. But Rigby explains that the reason he hates Don is because ever since they were kids, Don managed to take Rigby's friends away from him unintentionably because he was so cool, which drove Rigby to be heartbroken. Don apologizes for taking Rigby's friends from him, and explains he was just trying to be a follower of Rigby, because he believed everything Rigby did was so cool, but everyone just thought these were Don's ideas and that Rigby was his little brother, but it was the exact opposite. With Rigby and Don's relationship better, they go back to the park in order to stop the audit. Don is almost finished with the audit, but says that Rigby still needs to give him some sugar, so Rigby reluctantly does so under pressure. The park is restored to normal, and Don says good-bye, saying his work as an accountant is never done. Rigby and Don are now on better terms with each other, and Mordecai tries to offer Rigby sugar, but Rigby pushes Mordecai aside.