On Hearth's Warming Eve, Rainbow Dash *ahem* drops by Twilight's library. (Crashes. She crashes through the window like she does every time.) As punishment, Twilight tells Rainbow Dash about a book she had been reading by an author by the name of Doctor Colt Smith. The book tells of this holiday called "Christmas" that's celebrated in other universes. Rainbow Dash doesn't believe Twilight's stories, however. If Rainbow Dash was fast enough to fly around Equestria in 12 measly hours, how could 8 flying deer possibly come close? When Rainbow Dash voices her doubts, she and Twilight get a surprise visit not from Santa, but somepony who promises to prove Dash wrong.
Unfortunately, the two ponies and one madman end up injuring Santa Claus, putting Christmas on Earth in jeopardy. Now it's up to Twilight to pose as the big man and Rainbow Dash to strap on the fake antlers and glowing red nose so they can save Christmas and possibly save Santa's life.