Wiktionary
n. The primary plot of a piece of literature, television, etc., contrasted with the secondary B plot.
Usage examples of "a plot".
Northern Neck, a plot was discovered in which slaves planned to kill all the whites in the area and escape during a mass funeral.
The meeting called the war a plot by slaveowners and asked for the withdrawal of American troops from disputed territory.
The evidence for such a plot was very weak, coming as it did from the notoriously corrupt Kuwaiti police.
In other words, a sort of quirk of history, the kind Shakespeare drew on freely when he needed a plot or a character?
At times, in discussing how much of the substance of a play the copyright protects, courts have indeed used language which seems to give countenance to the notion that, if a plot were old, it could not be copyrighted.