Honkaku is a subgenre of Japanese Crime Fiction that came into being sometime in the early 1920's. The original definition was "a detective story that mainly focuses on the process of a criminal investigation and values the entertainment derived from pure logical reasoning". The term was coined to clearly differentiate Honkaku mystery fiction from other subgenres and it was used for both local and Western writers, although a distinct Japanese form became increasingly common in the 1950's.
Adding depth to long tradition, the author of THE TOKYO ZODIAC MURDERS, Soji Shimada has written over 100 mystery novels.
Knowing the background to the form isn't required to understand the structure of this novel, although it does help to realise that it is informed by such a rich tradition. Written in a number of parts or acts, THE TOKYO ZODIAC MURDERS firstly introduces the reader to a bizarre prologue - a note written in the mid 1930's by artist Umezawa outlining detailed plans and justifications for the murder of six of his daughters and nieces to create the figure of a supreme goddess Azoth. Disturbing and frankly very odd, the voice in this section is manic.
In the next part, 40 years on from the date of the note, the reader discovers that the six daughters referred to in the plan were indeed murdered, mutilated exactly as outlined. Which would have been a straight-forward case had one other daughter not predeceased her relatives, and Umezawa himself died before the girls. This is revealed as Kazumi Ishioka explains the background of the mystery that has baffled Japan since it occurred to his friend and self-styled detective Kiyoshi Mitarai. There investigation occurs in two parts from here - and along the way Mitarai accepts a threat / challenge which means that he must not only solve the puzzle, but do it to a schedule. To do that Mitarai and Ishioka travel through Japan, following where the clues lead them.
Those clues are laid out in a very traditional manner allowing the reader to discover them as the main detecting characters do. In an interesting touch the point at which the detective has solved the puzzle, the story switches to a note from the author letting the reader know that they now have everything needed as well. And it is actually solvable if you really pay attention, and take a logical and considered approach (this reader kind of worked out who, but admits it was more by good luck than good management, mathematical formula of any kind being the ultimate mystery after all).
Because the point of this style of novel is to present a puzzle, there's not necessarily a lot of character development alongside that, although the final resolution is markedly sad and moving. The detecting pair's travels around Japan do, however, give it a strong sense of place as well as many insights into the culture and customs of the country. The centrality of the Zodiac to the mystery does mean that some of the patterns - such as the dates of birth of the victims, the relationship of them to the structure of the Supreme Goddess to be built, and the layout of the closed room mystery in the murder of Umezawa are made more fathomable by the insertion of diagrams.
Puzzle upon puzzle THE TOKYO ZODIAC MURDERS is intricate and utterly fascinating, as an example of Honkaku and the culture from which it sprang, as well as providing sufficient puzzles to be solved to keep a reader happily occupied (and slightly confused) right to the last page.