After the narrator hangs his first black cat, Pluto, from a tree in the garden, knowing he is committing a truly vile sin, he discovers a similar cat in the bar where he often drinks. It is as large as Pluto had been, but Pluto was all black, and this cat has "a large, although indefinite splotch of white, covering nearly the whole region of the breast." Initially, then, the white patch has no discernible shape.
As time passes, the narrator begins to dislike this cat as well, and he soon grows disgusted with it. It, too, has only one eye, and it begins to like the narrator more and more. The narrator reminds us that the white mark had, at one time, possessed an "indefinite" shape, but, slowly, "it had, at length, assumed a rigorous distinctness of outline [. . .], the image of a hideous—of a ghastly thing—of the gallows!" Thus, the white "splotch" eventually evolves into a shape that looks like the apparatus by which one is hanged.