On the night of Hearth's Warming Eve, Sweetie Bell, Diamond Tiara, Scootaloo, Button Mash, and Silver Spoon are sent to Sweet Apple Acres by their parents, so they won't cause trouble while the rest of the inhabitants of Ponyville celebrate in the Town Hall. Marple Pie also comes, wanting to see Big Mac, whom she is in love with.
The four foals get into an argument about their parents and whether or Santa Hooves, the spirit of Hearth's Warming, is real. Granny Smith warns the foals not to fight, telling them the legend of Grogar; a monstrous, powerfully magical ram who lives in the faraway ruined city of Tambelon and only comes out on Hearth's Warming Eve, during which he steals away any ponies who fight each other or even forget the spirit of the holiday. The foals don't believe Grogar is real.
Applejack and Granny Smith leave the foals to bake cookies, while Big Mac and Maple Pie go elsewhere to talk. With the adults absent, the argument between the foals continues, with Silver Spoon mocking Scotaloo for believing in Hearth's Warming, and Sweetie Bell for her crush on Button Mash. As the argument escalates, the blizzard outside gets worse. Eventually, the argument gets to the point that Diamond Tiara ends her friendship with Silver Spoon. Enraged, Silver Spoon declares that she hates Hearth's Warming and everypony. Suddenly, something lands on the roof of the farm house and an icy cold wind blows down the chimney, extinguishing the fire. Seconds later, a hooked chain snakes down the chimney, grabs Silver Spoon, and pulls her away up the chimney, leaving the remaining foals screaming in shock and horror.
Applejack and Granny Smith come to see what's happening, and Button Mash explains. Seconds later, they hear the sound of a window breaking upstairs. Granny Smith seems familiar with the situation, and when Applejack goes to find Big Mac, warns her to stay away from the windows.
Applejack goes upstairs to Big Mac's room, only to find Maple Pie cowering under some bedsheets. A terrified Maple Pie faints, but not before claiming that "he" took Big Mac. Then, Scootaloo, who followed Applejack and overheard Maple's words, becomes terrified, believing Grogar is after them. The young Pegasus tries to fly out the broken window, but is grabbed by a hooked chain too and pulled away into the blizzard raging outside. Though horrified and saddened by the foal's fate, Applejack takes the still unconscious Maple Pie back downstairs, where Applebloom is heartbroken to learn of Scootaloo's fate.
Eventually, Granny Smith explains that the kidnapper is Grogar, who has come to claim everyponies' souls. She knows this because when she was a child living in Dream Valley (a different part of Equestria) Grogar took her best friend, Pumpkin Patch, after she had a fight with her on Hearth's Warming Eve. The ponies settle down to rest while Granny Smith rebuilds the house's fire and sings to keep Grogar at bay, as the evil ram hates singing. Meanwhile, Sweetie Bell and Button Mash confess their romantic feelings for each other and share their First Kiss.
Eventually, everypony falls asleep except Granny Smith falls asleep. But eventually, due to her old age and the warmth of the fire, Granny Smith stops singing and falls asleep too. As a result, Grogar manages to break in and take Granny Smith. In the following panic, Button Mash and Sweetie Bell flee upstairs, while the others rush to the barn outside. As she tries to flee, Marple Pie trips and hurts her leg, which enables Grogar to take her too.
Grogar then tries to take Button Mash, but is stopped by Sweetie Bell, who uses her magic to send Grogar flying outside. Undeterred, the evil ram corners Applejack, Apple Bloom and Diamond Tiara in the barn. Being the big brave sister that she is, Applejack offers to give herself up to Grogar if he spares Apple Bloom and Diamond Tiara. Grogar appears to agree, but after taking Applejack, he goes back on his word and prepares to take the two fillies as well. As Apple Bloom and Diamond Tiara prepare to meet their doom, they embrace, apologizing for all their conflict in the past. Unexpectedly, their friendship summons a powerful magic that banishes Grogar back to Tambelon until the next year.
A few minutes later, Silver Spoon appears... but it's not Silver Spoon... not really. Her eyes are dark pits, pouring with tears, her mouth is curled into a rictus grin. She has been transformed into an undead ghoul, enslaved to Grogar. Horrified by her friend's terrifying fate, Diamond Tiara gives Silver Spoon back her glasses (which she dropped when Grogar snatched her). This act of kindness briefly coaxes out the real Silver Spoon and she apologizes for her cruelty. Sadly, it doesn't last, and the enslaved, undead Silver Spoon quickly runs off into the cold winter night, never to be seen again.
The next morning, the four foals attempt to head back into town, but they discover, to their utter horror, that the entire town of Ponyville has been turned into a frozen wasteland, and all of the residents have vanished without a trace. Celestia arrives asking what happened, and Apple Bloom, now driven utterly mad by grief at losing her home and family, explains that Grogar froze the town and took everyponies' souls.
