Mark Twain was famous for his stinging wit, but he also had a softer side. The author was the father of two daughters, Clara and Susy Clemens, and every night, he told them a bedtime story. Twain never published any of these fairy tales, but he did outline one, and called the 16-page work "Oleomargarine." Now, The New York Times reports, publisher Doubleday is expanding the tale into an illustrated children’s book, slated for release this fall.

Researcher John Bird was perusing University of California-Berkeley’s vast Twain archives in 2011, searching for food references to add to a Mark Twain-themed cookbook. He thought "Oleomargarine" sounded like a recipe—but it turned out to be a bedtime story, written by Twain nearly 140 years ago.

"Oleomargarine" refers to a prince of the same name, who’s kidnapped by giants, and held hostage in a cave guarded by dragons. A poor boy named Johnny—who gains the power to talk to animals after eating a magic flower—embarks on a mission to rescue him.

After perusing Twain’s journals, which mentioned nightly bedtime tales featuring a boy named Johnny, Bird guessed that Twain told the story to his daughters in 1879 (third daughter, Jean, was born the following year). Susy and Clara eventually grew bored with the character, but nobody quite knows why Twain didn’t finish "Oleomargarine," nor whether he ever planned to share it. However, experts do believe it may have been the only fairy tale that Twain—a prolific journalist, essayist, novelist, and short story writer—ever committed to paper.

Bird notified the Mark Twain House and Museum in Hartford, Connecticut, and they sold the rights to Doubleday Books for Young Readers. The publisher hired author-illustrator duo Philip and Erin Stead to expand on the work; the final product is The Purloining of Prince Oleomargarine, a 152-page book featuring original drawings.

Twain's voice is always present, even though the book is framed as a tale "told to me by my friend, Mark Twain." Philip Stead read a few volumes of the author's massive autobiography to capture his natural tone, and in one scene, Twain even pops into the story and argues against the plot's progression.

The Guardian reports that The Purloining of Prince Oleomargarine will be published on September 26, which is the 150th anniversary of Twain’s first book, the short-story collection The Celebrated Jumping Frog of Calaveras County and Other Sketches (1867).

[h/t The New York Times]