The poem begins with a memory: a story the speaker's brother used to tell, a joke about a man who flings open his door and simply isn't there. Sounds are all that remain, a chirping bird, a yipping dog, a lock clicking open. It's funny in a strange, slightly spooky way.
But the poem turns on the line "I distress that will happen." The speaker takes the joke personally and fears becoming the man who isn't there when the door opens. That reads like a fear of dying, or of missing one's own life, though the poem never says so outright. The short, clipped lines near the end ("I unlock, turn, open / Dog barks / Not there") make the fear feel immediate, as if the speaker is acting out the story. The final line, "Not coeval with the door," suggests a gap in time: the speaker and the opening never line up. The mood is uneasy, quiet, and tinged with dark humor.