刺のある樹 (toge no aru ki, A Tree with Thorns, 1961) is the third novel featuring amateur detectives NIKI Yuutarou (仁木雄太郎) and his younger sister Etsuko, the narrator, who shares a name with the actual writer of the books, NIKI Etsuko (仁木悦子). I reviewed the first and second novels in the series earlier, as well as a collection of short stories. Picking up from the last novel, Yuutarou and Etsuko are still students house-sitting for a rich cactus collector who is currently living in Europe. The absent owner also conveniently allows them to use his Renault. Yuutarou is tall, thin, intellectual and more likely to observe social proprieties than short, active, Etsuko. Etsuko is an enthusiast for detective stories and by association for real crime investigation, Yuutarou shows less enthusiasm at first, but once involved in a case is more insistent on doing things his own way.
The new case starts with a visit from a businessman, ONAGA Masaji (尾永益治), who believes that he is being targeted by an unknown assassin. He and his wife have had several accidents in the last months which he thinks were murder attempts; but the police brushed off his concerns. The investigating inspector, who had worked with Yuutarou on earlier cases, had suggested that he talk to him if he was still concerned. Yuutarou agrees that there is certainly something to investigate; but when Onaga rings home to prepare his wife for the visit, he finds that she has been murdered. She had been strangled while bent over her sewing machine; and the maid, who heard her working at it, gives everyone connected with the case an alibi.
As in the earlier books, the mystery is mostly a comic detective story, with much of the humour coming from the narrator Etsuko, both as character and as observer. This kind of thing irritates some mystery readers, who look on busybodies approaching other people's tragedies with light hearted curiosity as sociopaths. A part of the humour is in fact Etsuko's inappropriate attitude here. But in the second half of the book, as a new murder takes place, the contradictions of the genre catch up with the characters and the tone becomes more serious.
As a mystery, it's well narrated with some skill in placing clues, but a fairly minor work.