The protagonist, a Japanese man from the 21st century, dies in an accident and is reincarnated as a mountain in a desolate, barren world. Initially confused and distressed by his new form, he eventually accepts his fate. He discovers he can sustain himself by absorbing heat from underground lava, which also causes his mountain body to slowly grow and form high-purity crystals at its core. His existence is solitary, marked only by the frequent meteor showers that bombard the planet. He theorizes this is Earth after humanity's extinction, a desolate wasteland where he is the sole survivor, a sentient mountain.
One day, the sky darkens dramatically, and a colossal meteor crashes into the planet, causing widespread destruction and turning the world into a molten inferno. The mountain survives, but the impact drastically alters the Earth's landscape, covering it in lava and ash. After eons of enduring this fiery state, the world begins to cool, and the mountain observes the formation of a ring of debris around the Earth, which eventually coalesces into a new moon. This realization shocks him, as it implies he has been reincarnated on an Earth far earlier in its history than he initially believed, with billions of years stretching before him in solitude.
To combat his loneliness, the mountain decides to create life. He uses his stored crystalline energy, amplified by the immense heat he has absorbed, to fashion dragons. These dragons are perfectly adapted to the harsh, hot environment, possessing immense durability and the ability to absorb heat for sustenance, granting them near-immortality. Initially, the two dragons he creates are content to live freely, but they soon begin to reproduce exponentially, filling the sky. The mountain, realizing the potential for overpopulation and resource depletion, imposes a limit on their reproduction. This restriction sparks a rebellion, and the dragons attack him, attempting to steal his core crystals.
Outmatched, the mountain creates a new being: a human-like protector named Saela, imbued with a powerful weapon, Mjolnir, designed to combat the dragons. Saela proves incredibly effective, decimating the dragon population. After the conflict, Saela, despite her immense power, expresses fear of abandonment and death, leading the mountain to vow never to destroy his creations again. He then creates dwarves to bury the fallen dragons, who, like him, thrive on heat and rock. The dwarves settle within his mountain, becoming his companions and artisans.
As the Earth's environment shifts, cooling and eventually experiencing a long rainy season, the dwarves adapt. The mountain then creates mermaids to explore the newly formed oceans, granting them the ability to live in water and derive energy from it. Later, during a global ice age, he creates werewolves, adapted to the cold, and harpies, capable of flying in the harsh winds. Each new race is given a leader with extended lifespan, and the mountain observes their development and interactions.
The narrative then details the planet's evolution through various cataclysmic events and environmental shifts: a period of intense volcanic activity leading to a superheated Earth, a global ice age, a period of extreme storms and tsunamis, and the eventual rise of dinosaurs. Throughout these epochs, the mountain and his created races adapt and survive. The discovery of "dragon crystals," remnants of the ancient dragons, allows for the creation of magical weapons and items, further diversifying the capabilities of his creations.
The story culminates with the rise of mammals and, surprisingly, birds as dominant species after the extinction of dinosaurs. The mountain witnesses the evolution of apes into humans, observing their development and the establishment of various civilizations. He notes how his own existence has irrevocably altered Earth's history, yet the broad strokes of evolution and environmental change remain familiar from his past life. The narrative concludes with the mountain, now a colossal peak on Antarctica, observing the ongoing evolution of life on Earth, a silent, eternal observer of its ever-changing history.
