Matango

In a Tokyo mental hospital, a group of doctors questions Professor Murai about the events that led to his confinement. Murai agrees to recount the voyage and explains that only two of his party died, yet he was the only survivor rescued. He begins the tale by describing their carefree yacht trip and the people aboard.

A sudden storm hits the yacht and threatens to capsize. The boat endures heavy damage and drifts off course. The crew's sense of safety dissolves as their isolation begins.

The battered yacht drifts to a seemingly deserted island. The group disembarks and begins to explore for shelter and supplies.

They come across ponds of fresh rainwater and a forest filled with unusually large mushrooms. The discovery hints at strange ecological changes on the island.

They find a wrecked ship with rotten sails and mold-coated interiors. Murai reads the ship's log and warns the others not to eat the mushrooms, since the former crew reportedly hallucinated after trying them.

In clearing the mold, they notice the substance can be killed with cleaning products, enabling them to explore the vessel. The group starts to connect the ship to nearby nuclear tests, suspecting fallout caused mutations in local flora and fauna.

As food runs low, Kasai refuses to help and isolates himself in the captain's quarters. The others grow anxious about their dwindling supplies and the possibility of abandoning the island.

One night Kasai is attacked by a grotesque man who vanishes after encountering the group, heightening paranoia among the survivors.

Drunk Yoshida decides to use the mushrooms' power to assert control, pulling a gun and declaring violent intent. The others subdue him and lock him in the captain's quarters, which ironically ousts Kasai from his refuge.

Naoyuki hogties Kasai and flees with the gathered food, including Koyama's hidden stash meant for extortion. The group reels from the betrayal as supplies vanish.

Mami frees Yoshida and they attempt to seize control of the ship, shooting and killing Senzō in the process. Murai and Kasai manage to wrestle the gun away and force the two away from the vessel.

Mami entices Kasai into the forest to eat more mushrooms; perpetual rainfall accelerates fungal growth. Those who have eaten the mushrooms begin transforming into humanoid mushroom creatures.

Murai finds the adrift yacht and a note Naoyuki left listing the dead, then marks a large X over it. Akiko is revealed to be under the mushrooms' influence and is kidnapped amid the chaos.

Murai tracks Akiko but is overwhelmed by the mushroom creatures and escapes back to the yacht. After several days, he is rescued and returns to civilization.

Back in the hospital, Murai reveals that his face bears fungal growth and contemplates whether staying with Akiko would have been better. He concludes that humans are not much different from the mushroom creatures.