In the middle of an upcoming blizzard, the Mesohippus of Mesoequestria are driven back to the surface. But to Equestria's surprise, so are other subterranean prehistoric creatures from the Pleistocene era of the First Windigo Ice Age. They are joined with aurochs, wooly rhinos, Irish elks, quaggas, deinotheres, paraceratheres, megaceropses, macrauchenias, toxodons, palaeotheres, platybelodons, gomphotheres, and the large proud mammoth, along with the leader King Hengetrunk and his daughter Gelisol, who gets the attention of Kopje. They explain that something underground has been destroying their underground homes. So while the Lodgers go beneath to investigate, the Equestrian heroes find a chance to make new friends from prehistoric past, which proves to be much more primitive than they believed. They learn that the underground land of Paleopotamia contains the soul of Prince Hypogean, the Alicorn god of subterra, dirt, and caverns, who explains that the caves are threatened by a long-living Windigo tyrant from the First Windigo Ice Age named Permafrost who orchestrated the extinction of the prehistoric creatures in Equestria in order to prove he was the perfect leader and provider for food to his people, orchestrating additional things to the PR of the creatures back then to start a race war in order to do so, but had been exiled in permafrost beneath the surface as punishment for these crimes. But when the inhabitants below accidentally released him, he swore to finish what he started and when they escape to the surface, he seeks to escape through the elaborate Lascaux Catacombs, which is easy to get lost in, and start a Third Windigo Ice Age. Now with the caves around them freezing and threatening to freeze them, and with the blizzard above beginning to start, it's a race to stop Permafrost before he escapes and uses the blizzard to start his new ice age.