It is an alternate history, set in a world where a gender-sensitive plague killed most of Japan's menfolk in the mid-17th century; the story itself is set a couple of generations later, in the early 18th century, in an era when men are prized as potential breeding stock but excluded from the levers of formal power. The first three of four issues collected here follow the story of Mizuno Yunoshin, a poor but good-looking boy who joins the Ōoku, essentially the harem of the shōgun, at a time of political change. (The fourth issue has the new shōgun looking into the archives and presumably setting up a framing narrative for historical flashbacks the next volume.)
It's a fascinating construction. This is a path that a couple of other writershave previously trodden, most notably John Wyndham in his story "Consider Her Ways" (where all men, rather than most, have become extinct). Apart from the information that men now become commodities to be traded on the marriage market, and that the plague has not affected other countries, most of this first volume simply looks at the inversion of gender relationships as applied to the shōgun's ōoku in our world. There's an extraordinary moment when the shōgun speaks to a visiting Dutch delegation from behind a curtain, so that they will not realise that she is a woman; and she then commands a historical exploration of why patriarchal nomenclature continues to be used. Indeed, although Mizuno Yonushin is the ostensible viewpoint figure of the first three issues, I found the new shōgun, Yoshitsune, much the most interesting character.
Anyway, I shall try and get hold of the remaining volumes - I see that the next three are available in English translation. Good for the Tiptree Award, for calling attention to fascinating works like this one.