A well constructed playlet plot is one whose Beginning states the premises of its problem clearly and simply, whose Middle develops the problem logically and solves the entanglement in a “big” scene, and whose Ending rounds out the whole satisfyingly— with a surprise, if fitting.
But, temperate and helpful as this statement of a well constructed plot may be, there is something lacking in it. And that something lacking is the very highest test of plot—lightly touched on at various times, but which, although it enters into a playwright’s calculations every step of the way, could not be logically considered in this treatise until the structure had been examined as a whole: I mean the formidable-sounding, but really very simple dramatic unities.
III. THE THREE DRAMATIC UNITIES
Now, but only for a moment, we must return to the straight line of investigation from which we swerved in considering the structural parts of a playlet plot.
At the beginning of this chapter we saw that a simple narrative of events is made a plot by the addition of a crisis or entanglement, and its resolution or untying. Now, the point I wish to present with all the emphasis at my command, is that complication does not mean complexity.
1. Unity of Action
In other words, no matter how many events you place one after another—no matter how you pile incident upon incident—you will not have a plot unless you so inter-relate them that the removal of anyone event will destroy the whole story. Each event must depend on the one preceding it, and in turn form a basis for the one following, and each must depend upon all the others so vitally that if you take one away the whole collapses. 
 See Aristotle, Poetics, Chapter VIII, and also Poe’s criticism, The American Drama.
(a) Unity of Hero is not Unity of Action. One of the great errors into which the novice is likely to fall, is to believe that because he makes every event which happens happen to the hero, he is observing the rule of unity. Nothing could be farther from the truth—nothing is so detrimental to successful plot construction. 
 See Freytag’s Technique of the Drama, p. 36.
Aristotle tried to correct this evil, which he saw in the plays of the great Athenian poets, by saying: “The action is the first and most important thing, the characters only second;” and, “The action is not given unity by being made to concern only one person.”
Remember, unity of action means unity of story.