The narrator examines a mysterious companion who mirrors his appearance, moods, and misfortunes so exactly that clothes, illnesses, and gestures recur between them; the other never speaks yet seems expressive, delights in bathing and odd public antics, and appears at the same private and public sites as the narrator. A collection of wry anecdotes raises uncertainty about whether the double is supernatural, a prank, or a psychological reflection, while probing themes of identity, sympathy, imitation, and the discomfiting intimacy created by an inescapable resemblance.