In 1914, as an undergraduate studying Old and Middle English at Oxford University, J.R.R. Tolkien wrote his first known work of serious prose fiction: a short story called “The Story of Kullervo,” based on a tragic hero from Finnish mythology.

Tolkien scholar Verlyn Flieger transcribed the unfinished story in about 2009, copying it from the author’s handwritten manuscript at Oxford’s Bodleian Library. She published it, along with two talks the author gave about it, in an academic journal in 2010, but she felt it deserved more attention. Now it will be published for the first time for mass audience, coming to the U.K. on August 27 and the U.S. in April.

“The Story of Kullervo” (pronounced COAL-ervo) is about a boy brought up in the homestead of a dark magician who has killed the boy’s father and kidnapped his mother. Tolkien’s version, which fills both sides of 13 foolscap sheets, breaks off just as Kullervo discovers that he has unwittingly seduced his twin sister, Wanona. On the final page, the author sketches an outline for the remainder of the bleak story.

The story was based on the Finnish Kalevala, which Tolkien read for the first time in 1911, Ms. Flieger said. It is clear from the manuscript that Tolkien’s character Kullervo is the literary ancestor, or source, for Túrin Turambar, the tragic incestuous protagonist of Tolkien’s “The Silmarillion,” said Ms. Flieger, a retired professor from the University of Maryland.

Tolkien himself identified this story the genesis of his world-building.