Geophysicist Dr. Tadokoro and Onodera Toshio command the submarine Wadatsumi-1 to the Ogasawara Islands to investigate tremors in the seafloor, only to uncover a chilling truth: the landmass of Japan is steadily collapsing into the Japan Trench. This discovery sets off a chain reaction that tests science, politics, and the resilience of a nation facing an almost unimaginable fate.
As the pair return to the mainland, they are drawn into a complex web of human connections. Abe Reiko enters the story, and she and the scientist’s ally become lovers in the shadow of rising danger. While the couple finds a moment of solace on a sunlit beach, Mount Amagi erupts, signaling that the earthquakes and volcanic activity threatening Japan are far from over. A high-level meeting of government officials, including Prime Minister Yamamoto, gathers to assess the mounting disaster, but Tadokoro’s urgent warnings are dismissed as alarmist by those who insist on waiting for more certainty.
In the midst of political indecision, Tadokoro meets a mysterious, wealthy patron, Mr. Watari, who agrees to finance ongoing research expeditions. With Watari’s backing, Tadokoro lays out a plan to confront a nationwide earthquake disaster, a plan that will require bold steps and hard choices. The crisis intensifies when a separate effort to secure a research submarine from France is interrupted by a fresh eruption of Mount Kirishima, underscoring the immediacy of the threat.
Onodera leaves his post as a submarine pilot to work alongside Tadokoro full-time, joining the broader search for answers. Their research confirms a terrifying reality: the archipelago is on a trajectory toward the sea. The moment of truth arrives with a cataclysmic Tokyo earthquake that devastates the city and costs millions of lives—3,000,000 deaths that mark a turning point in Japan’s history.
Months pass, and Tokyo begins the painful process of rebuilding. Yamamoto, now personally affected by the disaster after losing his wife, collaborates with Watari and Tadokoro’s team to forecast what comes next: another, even larger earthquake appears likely, threatening what remains of the nation. The stakes grow as Tadokoro unveils a two-phase strategy: D1, an intensified investigation of seismic activity in the Japan Trench; and D2, a coordinated evacuation of the Japanese islands.
Diplomacy and logistics move into the foreground. Yamamoto negotiates with other nations to accept refugees, while Watari outlines three possible paths for Plan D2—creating a new country, integrating Japanese refugees into other nations, or choosing to do nothing and accept Japan’s fate. Across the personal and public divide, Onodera wrestles with pain and purpose, and Reiko becomes a visible anchor in his life as he contemplates the future.
The Japan Meteorological Agency delivers crushing news: the original two-year timetable for Japan’s sinking has collapsed to a mere ten months. Immigration talks accelerate, though not all nearby nations are willing to participate; South Korea, China, and Taiwan resist. In tandem, shipping and air transport capacities are ramped up, and the government publicly proclaims Japan’s fate to the world. Onodera contemplates marriage to Reiko and plans to meet her in Geneva, yet a fresh earthquake triggers another eruption—this time Mount Fuji—that further fragments the already fractured country.
At a United Nations summit, plans are proposed for future resettlement sites as experts seek a lifeboat for the population. In the ensuing weeks, millions more Japanese evacuate, but the numbers fall far short of what would be required to save everyone. By the time Japan’s sinking accelerates, large portions of the archipelago—Kii Peninsula, Shikoku, and beyond—have vanished beneath the waves, leaving tens of millions displaced or deceased and a world watching as three superpowers pledge to take in refugees.
As the countdown nears its end, the story follows the human cost of choosing which lives to save and which ones to leave behind. Eleven days before the anticipated total submersion, Onodera toils in rescue efforts, clinging to the hope of reuniting with Reiko, while Yamamoto announces a halt to dedicated rescue operations as the situation becomes more hopeless. In a final, poignant moment, events unfold with Watari’s last conversation with Tadokoro and Yamamoto on his deathbed, the doctor’s quiet resolve to stand with his country until the end, and a helicopter lifting Yamamoto to safety as the vast landscape of Japan sinks beneath the ocean.
Months later, the aftermath is stark: Reiko travels through a European landscape by train in Switzerland, while Onodera moves through a crowded boxcar in the southwestern United States, both seeking an uncertain future in a world where the homeland has changed forever. The film closes on a note of endurance and cautious hope, emphasizing the human capacity to endure catastrophe even as it redefines a nation.
