late Old English plot "small piece of ground of defined shape," a word of unknown origin. The sense of "ground plan," and thus "map, chart, survey of a field, farm, etc." is from 1550s. Plat is a Middle English collateral form. The meaning "a secret, plan, fully formulated scheme" (usually to accomplish some evil purpose) is from 1580s, probably by accidental similarity to complot, from Old French complot "combined plan" (compare the sense evolution of plan), itself a word of unknown origin, perhaps a back-formation from compeloter "to roll into a ball," from pelote "ball." OED says "The usage probably became widely known in connexion with the 'Gunpowder Plot.' "
Meaning "set of events in a story, play, novel, etc." is from 1640s. Plot-line (n.) "main features of a story" is attested by 1957.