Pinkie Pie overhears Twilight, Rarity, Rainbow Dash, Fluttershy, Applejack, and Spike saying something that doesn't sit well with her. So she swims down to the bottom of the ocean (Bikini Bottom, to be exact!) and meets SpongeBob SquarePants and Patrick Star. Cosmo from Fairly Oddparents also ends up with them, and the four form a super awesome friendship!