In a hospital in Tokyo, a university professor named Kenji Murai is visited by a man who asks him about the events that led him to the hospital. The story is about a group of crew and passengers on a day trip on a yacht, including Murai; his shipmate assistant Senzô Koyama; writer Etsurô Yoshida; celebrity Masafumi Kasai, the owner of the yacht and two female passengers, professional singer Mami Sekiguchi and student Akiko Sōma. A sudden storm causes the yacht to nearly capsize, causing it to drift uncontrollably. The group arrive at a seemingly deserted island and begin to explore. They come across ponds full of fresh rain water and a large forest of mushrooms. The yacht's skipper, Naoyuki, warns them not to eat the mushrooms because they might be poisonous.