Robert Louis Stevenson wrote rhapsodically about his favorite penny dreadful, A Mystery in Scarlet (1866), by "Malcolm J. Errym," the pseudonym of "Sweeney Todd" creator and working-class London radical James Malcolm Rymer (1814-84), whom Stevenson considered a "genuine influence." Illustrated by the celebrated Hablot K. Browne ("Phiz"), it was serialized in Rymer's magazine The London Miscellany in 1866. This edition reproduces the text and illustrations of A Mystery in Scarlet for the first time since the nineteenth century and explores the serial's political context and impact on Stevenson.  Research for this project conducted at the Lilly Library, Indiana University, Bloomington was made possible by an Everett Helm Visiting Fellowship awarded by the Lilly Library in 2018.