Five years after Frisks escape from the Underground, she's returned to the goat family in search of a cure for her because her greatest enemy is back... Chara. Asgore, Asriel, and Toriel band together to help Frisk as she tries to restore each of the human souls in order for her to be safe.
Meanwhile, Sans is still mourning the loss of his dear brother, Papyrus, but soon, Papyrus returns as a ghost to warn him and the others that Frisk is back and that an evil force bouncing off of her is going to destroy their Christmas.
When the others, the goat family, and Frisk all meet up, it's a battle of who's who of heroes, the goat family or the others.
But soon, a force from Asriel starts forming too. Everyone knew that this could only mean that Flowey is coming back.
Undyne, Ghost Papyrus, Alphys, Napstablook, Sans, Mettaton EX, Asgore, Toriel, Asriel and Frisk all band together to save Papyrus, save the Underground and everyone in it, Christmas, and to destroy Chara and Flowey for good. But, the mission gets sidetracked as Asriel and Frisk start falling for each other. But before they turn evil, Asriel wants to tell Frisk how much he loves her.
Will the Undertale group be able to restore Asriel's and Frisk's souls, and save everyone, including Papyrus, or will they fail and everyone from both worlds, the humans and the monsters perish, and will Chara and Flowey ruin Christmas for everyone?! Read to find out in An Undertale Christmas!