そして五人がいなくなる (soshite gonin ga inakunaru, And Then Five People Were Missing, 1994) is the first book in the series 名探偵夢水清志郎事件ノート (Meitantei Yumemizu Kiyoshirou jiken nooto, Case Notes of Great Detective YUMEMIZU Kiyoshirou) by はやみねかおる (HAYAMINE Kaoru, born 1964). It's a lighthearted children's detective series, probably aimed principally at children around ten to twelve, although the narrator and her sisters are slightly older (about thirteen).
Yumemizu is the new neighbour of the IWASAKI (岩崎) family. He has moved into the ramshackle western style house next door and hung up a sign saying "Yumemizu Kiyoshirou, Great Detective". The children 亜衣, 真衣 and 美衣 (Ai, Mai, Mii, or I, My, Me, as they write their names) regard this declaration with suspicion; and Ai starts to investigate the detective, calling round on the new neighbour with a gift from her mother. Yumemizu is an eccentric, always dressed in a black suit and sunglasses, mostly lying around reading or sleeping. A former lecturer and self proclaimed great detective, his self confidence is boundless, but somehow hard to credit, even though he shows Ai his card, which reads "Great Detective Yumemizu Kiyoshirou".
"You showed me the card earlier, so never mind that. I mean, tell me what cases you've solved so far."
"Fine." But although Yumemizu's mouth stayed open, no more words came out.
"What is it?"
"I can't remember."
Seeing my suspicion filled eyes, he hurried to defend himself, "It's true. I really have solved any number of difficult cases. But when a puzzle's solved, it's not interesting any more and I just forget them."
In the end Yumemizu's deductions convince the sisters that he really is a detective; but since "Great Detective" isn't a real title, they end up calling him "Professor".
The first real case comes in the summer holidays. At a nearby amusement park, the performing magician "The Count" (伯爵) makes a young girl vanish from a box suspended on ropes above the stage. When neither he nor the girl reappear, the audience realise that she has in fact been kidnapped. The Count announces that this is the first of five people he is going to make vanish; and soon another three children disappear in impossible circumstances. The figure of the Count is very much like the villain of EDOGAWA Rampo's children's series; but here there is less adventure, more emphasis on the puzzle. The book aims to be a proper classical detective story, though one in a world in which plausibility is not really a criterion. There are a couple of good ideas in the various puzzles (and one nice use of a narrative trick), but most will seem a little obvious. The chief attraction of the book is in the humour, particularly in its eccentric detective and sarcastic narrator.