In the summer of 1894, John Addington and Henry Ellis begin writing a book arguing that what they call “inversion,” or homosexuality, is a natural, harmless variation of human sexuality. Though they have never met, John and Henry both live in London with their wives, Catherine and Edith, and in each marriage there is a third party: John has a lover, a working class man named Frank, and Edith spends almost as much time with her friend Angelica...