This volume continues the story of the first female shogun, Chie; her advisor, Kasuga, whom she inherited from her father, the last male shogun; and her lover, Arikoto, the former monk. The previous volume ended with Chie and Arikoto learning to accept and then love being pushed together by Kasuga, despite the barriers that they erected prior to entering and consumating the relationship.
The central theme of this volume seems to be the issues of succession and continuance of the family name. In particular the story deals with the difficulty that Lady Chie and Arikoto experience when he is unable to get her pregnant. The Kasuga has a solution but it is one that neither of the lovers likes. Accepting the gravity of the situation they learn to accept Kasuga's solution and eventually grow more closely together despite the wedge that Kasuga has places between them.
The larger context that the story takes place in is the destruction caused by the red pox plague, which not only kills people, mostly men, but also, more slowly, causes Japanese society to unravel. Chie is not unaware of the trouble the country is in. She knows something needs to be done but this is an unprecedented situation.
Paramount in Japan is the continuation of families and family names. With the loss of 80% of the male population, including the early deaths of most boys, this patriachal society begins to change. Women take over many roles previously reserved solely for men. Even the members of the nobility not struck down by the plague, all men, mostly older, are grudgingly forced to make their daughters their heirs.
Eventually the government headed by Chie and Kasuga accepts the need for change. Although this doesn't happen until one of her male advisors puts forth a proposal which eventually becomes law. The significance of the gender of the character who finally puts voice to what many others must have been thinking can be interpreted in a number of ways but what is most striking about it is that it shows that succession and preservation of the family takes precendence over other traditions, such as patriarchy. Thus, the law is changed and women begin to be legally recognized as heirs to titles previously held by male relatives and thus preserving their family lines.
This is not a comprehensive review. For a more in depth look at the series read Slightly Biased Manga's reviews of v1, v2, v3.