A daring polar expedition deploys a powerful submarine to breach the ice barrier and enter an under-world of perpetual twilight inhabited by lupine humanoids, primitive tribes and gigantic prehistoric beasts. The explorers confront sabotage, ambushes and savage stampedes, relying on ingenuity, duels and engineering to survive while investigating vaults, temples and ancient inscriptions. Encounters range from priestly cults to monstrous saurian combats and tidal disasters; the narrative traces their discoveries, rescues, betrayals and the desperate effort to escape back into sunlight.