Quarantined in a Tokyo mental hospital, a psychology professor named Kenji Murai, Akira Kubo, is visited by a group of doctors asking him about the events that led him there. Murai proceeds to explain how, despite only two of his party being dead, he was the only one to be rescued. He then relates the story of his day-trippers on a yacht: Murai, Masafumi Kasai, Yoshio Tsuchiya, the owner; Naoyuki Sakuta, Hiroshi Koizumi, the skipper; Senzô Koyama, Kenji Sahara, his shipmate; Etsurō Yoshida, Hiroshi Tachikawa, the celebrity writer; Mami Sekiguchi, Kumi Mizuno, a professional singer; and Akiko Sôma, Miki Yashiro, a student. A sudden storm causes the yacht to nearly capsize, and though the boat remains upright, it suffers serious damage and drifts toward a seemingly deserted island.
As they explore, they stumble upon pools of fresh rainwater and a forest filled with unusually large mushrooms, a sight that unsettles them but also offers a possible source of sustenance. They soon discover a wrecked ship on the shore, its sails rotten and its interior blighted by a mysterious mold. Murai studies the ship’s log and warns that the mushrooms might be poisonous, noting that the previous crew had hallucinations after eating them. The mold, they learn, is killable with cleaning products, which gives them a slim chance to salvage the situation. Their curiosity deepens as they connect the island’s troubles to nearby nuclear tests and a fallout that may have driven a bizarre mutation in local life, including the mushrooms themselves. Food runs low and tensions rise as desperation grows, with Kasai stubbornly refusing to help or share, choosing instead to linger in the captain’s quarters.
One night, Kasai raids the food stores and is attacked by a grotesque figure who quickly disappears into the forest. A drunken Yoshida decides to sample the mushrooms for their hallucinogenic properties and, after clashing with Koyama over Mami, declares violent intentions toward the women. The others subdue him and lock him in the captain’s quarters, an act that indirectly forces Kasai out of his isolation. Kasai then tries to convince Naoyuki to abscond with the food and the repaired yacht, but Naoyuki vehemently rebukes the idea. In a turn of events days later, Naoyuki hogties Kasai and flees with the gathered supplies, including Koyama’s secret stash, leaving Kasai to face an increasingly dire situation aboard the ship.
Desperation pushes Mami to free Yoshida, and together they attempt to seize control of the yacht, killing Senzô in the process. Murai and Kasai manage to wrest the gun away from Yoshida and force the two aggressors off the ship. Time passes, and Kasai is lured by Mami deeper into the forest where the perpetual rain has sparked wild fungal growth. He devours the mushrooms, hallucinates scenes of Tokyo nightlife, and sinks to his knees among the evolving humanoid mushroom creatures.
Murai discovers the yacht adrift and swims toward it, finding a note left by Naoyuki listing the group as dead and explaining that, having run out of food and energy, he has chosen to jump into the sea. Murai marks the note with a large X and continues his grim search. The mushroom creatures attack Akiko and Murai; she is kidnapped, and Murai tracks her down to learn that she, too, has fallen under the mushrooms’ influence, along with Mami, Yoshida, and Kasai. Murai makes a valiant attempt to rescue Akiko but is overwhelmed and forced to flee back to the yacht, escaping the island with his life intact.
Several days later, Murai is rescued and returns to a hospital bed, where he contemplates whether he should have stayed with Akiko on the island. The final reveal shows his face bearing fungal growths, leaving him to wonder aloud if it mattered where he was: humans, like the mushroom creatures, might not be so different after all. The story ends on a note of quiet ambiguity, inviting reflection on humanity, nature, and the thin line between civilization and organic transformation.
