Wheatley isn't really concerned with narrative poetry. Action isn't her thing; ideas are. She wants to praise things and talk about ideas, like Christianity, salvation, and history.
There's a little narrative in her poem though, when the speaker writes, "brought me from my Pagan land." So, the speaker is a slave that was brought from Africa to America—by "mercy." And it's mercy that converts the speaker to Christianity, which she knew nothing about in Africa. Although her race is viewed negatively, the second half of the poem takes a turn to reflect her belief that anyone, Christians—yes, anyone, even "Negroes"—can be saved by joining Christianity.
The end. Or is it? For some in-depth interpretations, read on to our line-by-line summaries.