From the introduction:
Of all the plays Shakespeare wrote, only 37 have survived. However, scholars are delighted by the recent discovery of a chest containing the script to Shakespeare’s 38th play: the missing work Cardenio [sic]. However, it is not a complete script; the chest contains only a selected number of “sides” (i.e. the lines for a specific character). The surviving sides where written for the notoriously picky actor Devin Twinshire, who insisted that all his sides be written on durable sheepskin parchment (while all the other sides, written on paper, have since crumbled). Devin was a versatile actor; he seemed to have played a number of characters in multiple different scenes, and thus we have the lines for at least one character per scene.