Discuss the notion of the plot being the soul of the tragedy in Greek tragedies.

It looks like you would like to discuss Aristotle's notion of the plot as the "soul of tragedy" in Greek theater. To aid our understanding of this notion, we'll turn to Aristotle's Poetics.
In his treatise, Aristotle proclaims that plot is the "soul of tragedy" and that character must stand in second place. To Aristotle, tragedy involves action. This is because tragedy is really an imitation of life, which consists of human action.
He asserts that every good tragedy must have six parts, and these are plot, character, diction, thought, spectacle, and song. Additionally, since tragedy involves action, the plot must constitute the chief focus of the play. It is through plot, after all, that the actions of the principal characters are displayed.
Thus, Aristotle contends that "without action there cannot be a tragedy; there may be without character." He then goes on to describe how a good plot should be structured so as to achieve the maximum effect upon the audience. To Aristotle, a plot is a thing of precision: it must have a clear beginning, middle, and end. The unity of plot is of great importance: any beautiful piece of work must be undoubtedly perspicuous (or clearly outlined).
Aristotle criticizes the "episodic" plot, where actions succeed each other in no particular sequence. Essentially, an episodic plot contains episodes (or chapters) that don't present a harmonious whole, with a clearly defined beginning, middle, and end. The progression of the story may not be linear, so to speak, and there is no "natural continuity" between episodes. Aristotle maintains that the plot must follow a logical "cause and effect" sequence in order to create the deepest impression on the audience. Since tragedies imitate actions that inspire both fear and pity, they must follow a complex plan.
Aristotle explains this clearly in his treatise. He tells us that a good tragedy should not be about a good man who experiences adversity. Such a story merely shocks us; it doesn't evoke both pity and fear. Similarly, a good tragedy never shows an evil man moving from adversity to prosperity. Again, such a story evokes neither fear nor pity.
Aristotle also contends that the downfall of a villain falls into the same category: sure, it's satisfying in a moral sense, but it hardly inspires both pity and fear. Aristotle then tells us what a perfect plot entails: it should outline the story of a man (who isn't necessarily a virtuous character) and whose downfall is caused not by vice but by a "frailty" of sorts. Aristotle contends that it would be even better for this character to come from a "renowned and prosperous" background. He then gives us the examples of Orestes, Oedipus, Alcmaeon, and others.
So, to Aristotle, the plot is the "soul of tragedy" because it is the principle part of the play. Without plot or action, there can be no story to tell. So, plot must reign supreme over the other aspects of a play.
http://classics.mit.edu/Aristotle/poetics.html