* If "plain" refers to a flat area of land, the homonym is "plane", which refers to a flat surface or a flying machine.
* If "plain" refers to something simple or unadorned, the homonym is "plane", which refers to a level of quality or a surface with no projections.
For example:
* "The plain stretched out before them." (flat area of land)
* "The pilot flew the plane through the clouds." (flying machine)
* "She wore a plain white dress." (simple or unadorned)
* "The architect drew the design on a plane of paper." (flat surface)
So, the specific homonym will depend on the meaning of "plain" in the sentence.