The Undying Fire is a science fiction novel by Fletcher Pratt.

[1] The book is an expansion of the author's novella "The Conditioned Captain,"[1] originally published in the magazine Startling Stories in the issue for May, 1953.

[2] The story involves an interstellar expedition to steal a neptunium motor from the planet Danaan, punctuated by various political and romantic complications.

Groff Conklin, writing in Galaxy Science Fiction, called the book "tenuous," rating it "far below the best that Fletcher Pratt can do."

He noted that "[t]he astropolitics encountered are involved and the various love motifs energetic, but neither do much to rescue the book from its basic weakness--lack of inventiveness.