John D. Swain (1870–1952) was a prolific pulp author, playwright, novelist, poet, essayist, and movie script writer. He won the O. Henry Memorial Award in 1931.

His story “The Man Who Knew Too Much” is a fantastic tale originally published in Black Mask magazine. Although it became known as the home of hardboiled mystery writers, in its early years Black Mask published all manner of tales, including this one, which—had in been written in later years—would have been at home in either Weird Tales or Amazing Stories.

Includes an introduction by John Betancourt.