Synopsis
The movie begins in Tokyo where a man travels to visit a professor who is being held in the psych ward of a hospital. He tells the man that what happened to him sounded crazy but he in fact was not insane.
A Japanese Yacht on a day trip comes across a nasty storm that nearly capsizes the group, its crew and passengers are the skipper Naoyuki, his shipmate assistant Senzô, Etsurô Yoshida a writer, Kenji a university professor, Masafumi Kasai a celebrity and owner of the yacht along with two female passengers Mami a professional singer and a student Akiko. The storm leaves their ship in ruin, without a rudder or sails to steer by they are forced adrift. A few days following hearing a radio announcement that they were lost at sea, they come to a seemingly deserted island. After spending a day in search of food and water, they come across ponds that seem man made full of fresh rain water along with a seemingly endless forest of mushrooms. However Naoyuki warns them not to eat the mushrooms as they may be poisonous.
As they cross the island they come across a ship wreck on the shore, though it seems to have only been there about a year the sails are rotted and the ship's interior is covered with a mysterious fungus and mold that has spread throughout. Seeing a weakness to strong cleansing products, they work to clear the mold from the ship, uncovering the suspicion that the ship had been involved in some sort of nuclear testing of the polluted waters, forcing gross mutations on various objects including mushrooms. As the days pass, the group begins to grow restless as their supply of food stores start to run low, they instead try for turtle eggs and birds, though it is proven difficult as birds seem to actively avoid the island. With Kasai refusing to help find a way off the island and instead stealing from the food stores, Yoshida begins to get on edge, eventually eating from the mushrooms on the island instead of eating the potatoes and seaweed they are able to find to sustain themselves.
One night, as Kasai is raiding the food stores he is attacked by a grotesque looking man who promptly disappears after coming across the group leading them to believe something is very wrong with the island. Shortly after Yoshida and Kasai fight over Mami's affections, a craze comes over Yoshida as a direct result of the mushrooms' influence and he pulls a gun on the men. When he is locked in Kasai's room. Naoyuki decides that they must leave the island in order to survive, but the others don't agree so he departs on his own. Mami frees Yoshida and they attempt to take over the ship, shooting and killing Senzô in the process. Kenji and Akiko manage to wrest control from them and force them off the ship, forcing them to leave off into the island. Kasai travels out to the Yacht only to find Naoyuki missing and a note behind explaining he is responsible for the deaths of the group before jumping overboard himself. On his way back he is confronted by Mami who entices him to follow her into the forest, as perpetual rainfall had caused wild fungal growth, he finds that those who had been eating the mushrooms in fact start to turn into mushrooms themselves, and due to its addictive nature no one can escape once they take a bite. Kasai is last seen collapsing as mushroom beings swarm in on him.
Meanwhile, Akiko and Kenji are attacked in force by the mushroom people and they are separated and Akiko is kidnapped. As Kenji tracks her down, he discovers that she had been fed mushrooms and is under their influence along with Mami, Yoshida and Kasai. Kenji attempts to rescue Akiko but he is overwhelmed by the mushrooms and flees without her, making his way onto the yacht and escaping the island. The story concludes that several days had passed before Kenji was finally rescued, and as his story ends he begins to ponder if he should have stayed with Akiko on the island before turning toward the audience, his face covered in fungal growth as he exclaims that it wouldn't have made a difference if he had stayed or not, but he would have been happier there with his love. The screen fades as Kenji expresses that humans are not much different than the mushroom people as the camera pans over a night lit Tokyo.
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