A storm blows a yacht carrying young Japanese people to a deserted island. The only food is what little they can scavenge from a recent shipwreck, as nearly the entire island is covered with weird fungal growths that make all birds and animals avoid it.
One by one they give in to hunger and eat the mushrooms, which are delicious but highly addictive. The mushrooms mutate whoever eats it into mushroom people.
All but one of the group succumbs, turning them into freaky giant mobile mushrooms.
A lone survivor escapes the island. He's rescued and returned to Tokyo but at the very end we see half his face is covered in the fungal growth.