It's based on a Japanese SF story that itself was inspired by a 1907 short story by William Hope Hodgson, called "The Voice in the Night". It's also a cult classic. It's regarded as one of Ishiro Honda's best films (and he also made the original Godzilla). Its scriptwriter thought this was his best ever screenplay and did all his subsequent work under a pseudonym because he liked it less.

I see what they mean. The English title and the movie's posters suggest B-movie trash and you could easily persuade yourself that it is, but it's primarily a dark human drama. Seven people are on a yacht. Lots of money, lots of free time, lots of ocean. The crew regards the passengers as playing around on their fathers' money, but the music over the opening credits is jaunty and everyone gets on well... until a storm hits. Twenty minutes of fraught screen time later, our heroes have no mast, radio or engine, but at last they've sighted an island. Landing there, they find lots of mould, moss and fungus. There's another ship, but it's a wreck and there's no sign of its crew. The cleverest of our heroes infers that it's to do with the mushrooms and tells everyone not to touch them.

Unfortunately, though, they've only a week's worth of food. Will they cooperate intelligently? Hell, no. Some of them will try, but there will be theft, food hoarding, attempted murder, extortion of huge amounts of now-worthless money and the sexual objectification of women (and one of those two women likes having men fight over her). The mushrooms wait quietly in the background. Their time will come, but they're not important right now. This is primarily a story of friends falling apart and becoming selfish idiots, or worse. They were fine when everything was going well, but on the island there's only one trajectory as, bit by bit, people discard their principles and look out for number one.

Ishiro Honda told his cast that this was "a serious drama picture, so please keep this in mind and work accordingly".

The Matango themselves are an unusual kind of horror. They're not even evil. They're just mushrooms. They might even have been friendly if they weren't being shot at. They are, though, horrifying. This film was nearly banned in Japan because the Matango make-up reminded people of survivors of the Hiroshima and Nagasaki nuclear bombs. (That make-up and its associated special effects are still superb today, hitting a note of bold kitsch that works like gangbusters and makes them genuinely creepy.) Furthermore, people who get Matango-ised usually chose that fate by knowingly indulging in the mushrooms. You could read the film as a 1960s drugs metaphor (especially the final thoughts in Tokyo).

It's classier than I'd been expecting. It's also atmospheric and slow. The characterisation's dark and there are no jokes whatsoever. Contemporary Japanese critics disliked it, partly because it was not "a typical Japanese mainstream movie at all". It's definitely worth a look.