The word plot has no known origin and exists solely in English. The noun dates from the late 10th or early 11th century and originally meant “a small piece of land or area of ground.” Plot in the sense “a small piece of land in a cemetery” was originally an Americanism and dates from the mid-19th century.
In the mid-16th century, plot was used to refer to a map, ground plan, sketch, or written outline. At about the same time, it also came to mean “a secret, usually evil plan”; the verb meaning “to plan secretly, devise” comes from that sense of the noun. Plot in the sense “a storyline or main story of a play or novel” dates from the early 17th century.