“At the Mountains of Madness” follows a 1930s Antarctic expedition led by geologist William Dyer, who recounts a suppressed and terrifying discovery in hopes of preventing future explorations. As Dyer and his team push deeper into the unexplored continent, they uncover a range of ancient, nonhuman fossils and impossibly old geological strata that defy scientific understanding. A secondary party ventures farther and disappears, prompting Dyer and a graduate student, Danforth, to search for them.

Their pursuit leads to an immense, cyclopean city carved into the mountains themselves, its architecture alien and breathtaking in scale. Within its labyrinthine corridors, the two men piece together the history of a vanished extraterrestrial species known as the Elder Things, learning how they created life on Earth, warred with their own creations, and eventually succumbed to forces even more monstrous.

As they push deeper into the black heart of the city, Dyer and Danforth come face-to-face with an entity beyond comprehension, an encounter that shatters their sanity. Dyer’s narrative becomes a desperate plea to prevent anyone from returning to the frozen continent, for he fears that awakening the ancient horrors slumbering there would doom humanity.