When Frisk dived back down to Mt. Ebott to see a friend of hers, she was in for a voyage she'll never forget.

After freeing the whole monster race from Mt. Ebott, life was as good as it can ever be. Humans and monsters slowly got along once again and repaired the bond between both kinds.

A year after her journey, she dived back down into the cave, to see the only monster she didn't get to save, and her beloved friend, Asriel Dreemurr. However, she instead found herself diving into a mysterious portal just before landing.

As she woke up, she finds herself, with hooves and a horn, and comes face-to-face with the land of Equestria.