This is a combination monster/psychedelic mushroom Japanese movie. It starts off with a group of individuals going off on a boating trip on a yacht owned by a rich man. I never understood why some of the individuals were even on the ship! Soon all wished they weren't on it, though, because a vicious storm approaches. At one point, before things got really bad and the captain and crewmate were looking out at the horizon in the dark, I must confess I wondered if Godzilla was going to slowly start rising out of the water. No Godzilla, but some great scenes of a ship being tossed around in a storm. The best part of the movie, actually.
Adrift after the storm, the yacht eventually drifts towards a deserted island. There is an old deserted ship on the island, which the castaways discover was a research ship possibly studying something nuclear. (While the movie was made 18 years after Hiroshima and Nagasaki, obviously the nuclear theme still was coming up in Japanese monster movies.) Water is soon found, but not food, and the rest of the movie basically explores human nature, and how desperation affects individuals. This includes some adult themes about lust, as well as showgirl fantasies or flashbacks or hallucinations or whatever they were.
Hence, I don't see this as a kiddie monster film, especially since the mushroom monsters actually appear only for a short period of time in the movie. I think a kid would be bored most of the time watching this film. The ending goes back to the very beginning, where the lone survivor is in a hospital talking about what happened. Having that in the beginning, of course, gave away how many of the castaways made it. So, there is no suspense at all about that. What was the purpose of this movie? Obviously not to scare anyone with monsters, except for those who get scared very, very easily.
Maybe the movie was supposed to be a warning about what psychedelic mushrooms can do to the human brain, and how they can destroy a human life. Or maybe it was to show that humans don't need drugs to be greedy and self-serving. The two women in the film certainly don't fare well, and both end up acting like seductresses, even the one who was no such thing until eating the mushrooms. As a monster movie, I would not recommend it, unless you desperately want to see a monster movie you have never seen, and there is no other choice. As a study of human nature film, I also wouldn't recommend it, because it seems too cynical and nihilistic . It is watchable, though, and is dubbed pretty well, so there are no subtitles to read.