A speaker walking under streetlights notices a dark figure that seems to follow him. At first the figure feels threatening, "black, stout like a thug," and the speaker is wary, trying to stroll away as if nothing is wrong. But the figure keeps changing. It stretches ahead, fades into doorways, then returns under the next light. The reader soon realizes it is the speaker's own shadow.
The mood shifts from nervous suspense to quiet tenderness. Once the speaker pauses and understands, he calls the shadow "an unobtrusive friend," the only one he can call his own. That admission suggests loneliness. The comparison to "an omnipresent God" hints at how constant and unavoidable the shadow is. The ending is sad and surprising: he weeps for the shadow and worries what will happen to it when he is gone, which really shows his own fear of being alone and of dying.