The novel takes place in the era of Cromwell and Charles I, but the characters deal with the English Civil War which is coeval with an Industrial Revolution.

Prince Rupert is taken by the Roundheads; held captive at a country house, he falls in love with his captor's niece, Jennifer.

Rupert, fleeing Roundheads, finds refuge in a magical inn, The Old Phoenix, which proves to be a nexus between parallel worlds.

Holger Carlsen is another guest, born in a world where the Matter of France is history, and later trapped in "our own" twentieth-century America (the hero of Anderson's Three Hearts and Three Lions).

Valeria explains what will happen in the English Civil War in "our" timeline, including the king's execution, strengthening Rupert's determination to change events here.

At the Old Phoenix, Valeria believes that even if "romantic reactionaries" like Charles I won the English Civil War here, there is still the prospect of technological advance in North America.

Lester del Rey found Anderson's invention to be "a lovely conceit" and reported the novel to be "a fantasy I can recommend with pleasure.