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A party planner named Cheese Sandwich is hired to plan the party for Rainbow Dash's "birth-aversary," leaving Pinkie Pie feeling rejected.

An elated Pinkie Pie is planning the biggest and best birthday party ever for Rainbow Dash, but when a strange new party planning pony named Cheese Sandwich rides into town, plans change. Soon, everyone is so excited about Cheese Sandwich's party that they forget about Pinkie Pie, causing her to question her own talents and abilities; however, Pinkie Pie's pride won't let her go down without an amazing, daring duel. Special guest (voice) appearance by 'Weird Al Yankovic'.—Brony Dad

in the episode's opening, it shows a party at Appleloosa with a dark mysterious far away cowboy pony figure walking away whispering in a deep voice to his pet rubber chicken Boneless to throw another party this time in Ponyville.

Pinkie Pie is eagerly planning out Rainbow Dash's "Birth-Iversary" (an event that is both the rainbow pony's Birthday, but also the anniversary of when she first came to Ponyville) party. However, the festivities are put on hold when a lanky pale macaroni orange stallion with long curly brown mane named Cheese Sandwich appears. Just like Pinkie, Cheese claims to be a "Super-Duper Party Pony," but many are soon enamored by many of the features Cheese speaks of in regards to what he can do and appears play the accordion and do many other things which Pinkie can do but many seem to be more attracted towards him. Pretty soon, almost all of Ponyville is eager to see what Cheese can do for the Birthiversary, but Pinkie grows upset and rejected that she seems to have been replaced.

She attempts to find other ways to use her talents, but when she recalls how her parties always made others smile, she breaks out of her funk, and challenges Cheese Sandwich to "An Equestrian Goof-Off".

With Rainbow Dash judging, Pinkie and Cheese face off, with the winner headlining Rainbow's party, with the loser bowing out.

Both of the contestants use polka medley accordion music and all sorts of props, attempting to top the other, but when a giant rainbow-colored pinata falls on Rainbow (and leaves her with a frustrated look), Pinkie calls off the Goof-Off, claiming that Cheese wins by forfeit.

Shortly thereafter, the other ponies note Pinkie attempting to leave town, but tell her that she's overreacting. While Cheese is impressive, they still value her party making, and claim Ponyville is fortunate for having it.

It is then that Cheese Sandwich appears, and makes a confession. He tells how when he was little, he was so shy that no one knew his name. One day on a journey, he stumbled onto Ponyville, where a major party was being thrown, and was inspired to become a party pony, like the little filly who was throwing the party: Pinkie Pie!

Both Cheese and Pinkie then combine forces to hold the Birthiversary party, and everyone has a great time. With the party a big success, Cheese then heads off for another town to bring his talents to and gives his chicken, Boneless, to Pinkie as a "little memento" gift from his visit.