It is an alternate history describing the world as it would have been had John Brown succeeded in his raid on Harper's Ferry and touched off a slave rebellion in 1859, as he intended.

As a result, instead of the American Civil War, the U.S. faces a full-scale slave revolt throughout the South helped by a handful of white sympathizers and various European revolutionaries such as Giuseppe Garibaldi, and an invasion by Mexico, which seeks to regain the territory it lost in 1848.

Later, the black state (named "Nova Africa") becomes Socialist, touching off a whole string of revolutions and civil wars in Europe.

However, the story of the protagonist, a young black woman grieving the death of her husband on an earlier Mars mission, is mainly the framework for excerpts from the vivid diaries of two people who lived through the stirring events of 1859 and its aftermath – her ancestor, who was then a young black slave, and a white Virginian doctor who sympathized with the rebellion.

David Pringle rated Fire on the Mountain three stars out of four and described the novel as "a skilful evocation of an unlikely alternate history".