The novel is essentially an examination of the sexual mores and behaviors of the members of a small 19th century French village some time shortly following the Franco-Prussian War. The plot concerns a feud that has taken place for generations between the Hadouin and Maloret families, and a missing letter that contains revealing secrets relating to the conflict. The green mare of the title is in fact a magical painting of an unusually-colored horse owned by one of the Hadouins that has somehow been imbued with an observing consciousness by the artist who painted it.