Kindaichi has been enlisted and serving in the war. A fellow soldier and friend, KITOU Chimata (鬼頭千万太), is returning with Kindaichi to Japan, but dies on route, leaving Kindaichi with an entreaty.
I don't want to die .... I .... I .... don't want to die. If I don't get home, my three sisters will be killed. .... But .... I'm done for. Kindaichi .... in my place ..... in my place, go to Gokumontou for me.The Kitou family is the richest family on the little island. Normally they would be controlling the island's business; but the head of the family is recently deceased and Chimata's father is insane, kept in a locked part of the house. There is a branch family, too, still on speaking and visiting terms, but regarded as secretly hostile and plotting against the main family. The sisters that Kindaichi has come to save are strangely childish and lacking in empathy. And is the figure seen lurking around the house the cousin reported to be on his way back from the war? Or one of a group of discharged soldiers who had been practising piracy in the area? Or perhaps both? Soon one of the sisters has been killed, the first of a series of murders, each bizarrely staged, particularly the second one, where the victim is beneath a giant bell. (The bell was being returned to the island's temple after narrowly escaping being melted down for metal during the war).
The atmosphere, both of Japan immediately after the war and of the isolated island, is well managed. Fantastic elements seem to become more prominent than in The Honjin Murder Case, as they are in the later books.
Considered as a puzzle mystery, it's difficult to know how to judge it. There are sufficient clues to know roughly what was going on for at least two of the murders (the first is probably too obvious); but the larger story of what was going on would probably be hard to work out. On the other hand the larger story makes an impressive answer to the puzzle when you read it. Some Yokomizo stories, including some regarded as classics, turn out to have a vast amount of pointless complexity in the solution. Here there is a surprising and horrifying simplicity, to which everything goes back.
Gokumontou has been filmed and televised many times, most famously as part of the series of Kindaichi films by ICHIKAWA Kon (市川 崑) in 1977. I haven't seen it, from this discussion, it seems to have been similar to the original, with slight changes.
Incidentally, my discussion here is written from memory, and I no longer have my notes on the readings of the kanji for the names of the various characters. But if you want to prepare yourself, the book's Japanese Wikipedia page lists the characters with their readings. (The description of the story there currently tries to avoid spoilers, but you might still be better off skipping it and the descriptions of the various adaptations.)