It might be a challenge to find a specific plot to any poem as there is a collection of images that is more dominant. In Hughes' case, this is even more valid. His poem seeks to explore what it means to be Black in America and the multiple implications it has on the notion of experience. The hyphenated American experience is one that encompasses the promises and realities of America. Hughes explores this in a very poignantly personal and powerful manner. He is ahead of his time in his ability to point out the nuances of multiple experiences without beating it to a point where it loses effectiveness. Examine how Hughes describes the experience of living in Harlem and how this will impact what defines truth and then examine how he articulates the similarities and differences between the instructor and himself, in one being white and the other black. Hughes is able to evoke the essence of what it means to be different, its joys and sorrows.
There is really not very much of a plot to this poem. It is much more about a theme or idea than it is about a plot or set of events.
The only thing that actually happens in the poem is that the speaker's English teacher assigns him a "theme" or essay. The speaker is supposed to let the essay "come out of" him.
The speaker does that, we do not know where. In his essay, he reflects on the way that being black in America makes him sort of different from his teacher, but not as different as people might think.