大暗室 (Daianshitsu, The Great Darkroom, 1938) is a thriller by EDOGAWA Rampo (江戸川乱歩). I've not read any of his novels except for a couple of children's adventures, The Fiend with Twenty Faces (1936) and The Boy Detectives' Club (1937). Rampo's writing has quite a range, including children's stories, classical detective stories and "erotic grotesque" stories, which vary in tone from satire to horror.
I don't know where The Great Darkroom belongs in all this, but it's basically an adventure story with a supervillain and a righteous hero combatting him. In many respects it's very similar to the children's stories I mentioned above; but there is quite a lot that would not be acceptable in a children's book, to put it mildly. While in those the antagonist is above spilling blood, the villains in this readily murder women, children and innocent bystanders. There are occasional scenes of sadism, including sexual threats and threats of torture. A small child tortures a puppy in the prologue. The mix of horror and adventure elements is similar to many comics aimed at older children (of the kind that used to attract congressional investigations in America); and it is easy to imagine the readers of the children's stories graduating to this as they become teenagers, but probably without their parents' approval. Apparently there is a rewriting of the book (made after Rampo's death) in the Boy Detective's Club children's series, with AKECHI Kogorou vs. the fiend with twenty faces. Presumably the more offensive elements will have been removed as well.
The first section of the book is a long prologue set twenty odd years before the main events. Adventuring baron ARIAKE Tomosada (有明友定), his friend OOSONE Gorou (大曽根悟郎) and his servant Kurusu (久留須) are adrift in a lifeboat after escaping from a sinking ship. Sure that he will not survive Ariake writes a letter to his wife asking her to marry Oosone, who he hopes will look after her if she becomes a widow. When they spot land unexpectedly, Oosone, who has kept two bullets for his pistol, takes the opportunity to marry a rich and beautiful widow by shooting the other two passengers. A few years later they are married with two children, Ariake's son Tomonosuke (友之助), born after his last departure from his wife, and Oosone's infant son Ryuuji (竜次), who seems to have inherited his father's evil nature. When Kurusu unexpectedly returns from the dead, Oosone attempts to kill the whole household while making his escape with the family's transferable wealth and his son. But Kurusu again survives, though horribly disfigured by fire, along with Tomonosuke, whom he raises in secret to be the righteous avenger of his parents.
With that we move forward to present day Japan where two young men, going by the names ARIMURA and OONOKI, have won a great reputation for their varied ability and daring. When they meet, unaware of each other's true identity, they become a kind of friendly enemies. As Ryuuji starts setting his megalomaniac plans into action, Tomonosuke is there to combat him, mostly with mixed success, so that the villain gets away each time.
The book is much more interested in its villain than its hero. Like a Bond villain, Ryuuji has a vast underground base of interconnected caverns; and the narrative stops for fifty pages towards the end of the book while he gives captive journalists a tour of his kingdom. This is both unpleasant (megalomaniac villains are not people you want to spend time with) and tedious; and I imagine the original series readers must have been wondering when they could get back to the story.
Compared to the two children's books I mentioned in the first paragraph, the plotting of some of the episodes is allowed a little more complexity, although they still work on the same principle of constant surprising reversals. In particular the kidnapping of the "prince" in an all female musical revue is written as a kind of impossible crime. The description of this theatre genre is surprisingly similar to the way that the same thing is described today in the case of the famous Tarazuka Revue, with a club of fans, all young women, devoted to the protection of their idol.