Aristotle elaborates on what he means when he says that the action of a tragedy is complete in itself and with magnitude. For a plot to be a complete whole, it must have a beginning, middle, and end. The beginning is a point that does not necessarily follow from anything else, which naturally has consequences following from it. The end is a point that naturally follows from preceding events but does not have any necessary consequences following it. The middle is a point that is naturally connected both to events before and after it.
The magnitude of a story is important, as it is in any art. Paintings are neither infinitesimally small nor monstrously big because they must be of such a size as to be taken in by the eye. Similarly, a tragedy must be of a moderate length so as to be taken in by the memory. Usually, time limits are set by the audience or other outside factors, but Aristotle suggests that the longer the play the greater the magnitude, provided the poet can hold the tragedy together as one coherent statement. As a general rule of thumb, he suggests the action should be long enough to allow the main character to pass through a number of necessary or probable steps that take him from fortune to misfortune or vice versa.
In insisting upon the unity of plot, Aristotle makes it clear that he does not mean that it is enough to focus the plot on the life of one individual. Our lives consist of all sorts of disconnected episodes, and the story of a man's life would rarely have the completeness necessary for a unified plot. Rather, the poet must select some series of events from a character's life—as Homer does in the Odyssey —and craft them into a coherent whole. Any part of a story that could be added or removed without any great effect on the rest of the story is superfluous and takes away from the unity of the piece.
Aristotle distinguishes between poetry and history, saying that while history deals with what has been, poetry deals with what might be: it presents the possible as probable or necessary. Poetry is superior to history because history always deals with particular cases while poetry can express universal and general truths. Tragedy gives a feeling of necessity—or at least probability—to the way certain characters behave in certain situations and thus gives us insight into general principles regarding fate, choice, and so on. The worst kind of plot is the episodic plot, where there is no seeming necessity or probability whatsoever between events.
As a medium that arouses pity and fear, tragedy is most effective when events occur unexpectedly and yet in a logical order. The ideal is to have the audience see the final outcome of a tragedy as the necessary consequence of all the action that preceded it, and yet have that outcome be totally unexpected.
Essentially, a good plot is a complete causal chain that leads, with necessity or probability, from beginning to end. The beginning is the first link in a chain that is itself not necessarily caused by any events that precede it. The events that follow are necessary or probable consequences of this un-caused beginning. Each event follows the next until we arrive at the end, which is also a necessary or probable consequence of all the events that have preceded it. This end does not itself cause any further events with any kind of necessity or probability and so concludes the causal chain.
What kinds of plot does this definition exclude? Aristotle explicitly condemns the episodic plot, where one event follows another without any clear connection. Obviously, no plot is entirely episodic, though we could also say that very few plots are so tightly organized as to tie in every moment with seeming inevitability. The plot with a fully integrated beginning, middle, and end is an ideal to be approximated rather than an easily attainable goal.
That the plot of a tragedy should consist of one uninterrupted causal chain with no superfluous elements (nothing that is not a necessary part of this chain) is the essence of what Aristotle means when he talks about the unity of plot or action.
Again, we should be clear that the Greek mythos is not quite the same as the English "plot": we are not so much talking about the sum total of the events in the story so much as the way they are held together to form a coherent statement. If we were thinking simply in terms of the events taking place on stage, it would be obvious that a tragedy must have a beginning, middle, and end. In talking about a beginning, however, Aristotle is not talking about the first things that happen on stage so much as the first link in a causal chain that leads logically to the conclusion.
We might come to a clearer understanding of the unity of plot if we examine Aristotle's contrast between tragedy and history. Aristotle seems to hold the point of view that history is one thing after another. Event follows event, and there does not always seem to be a connection between them. This view is contestable, to say the least: the job of the historian, to a large extent, is to uncover some sort of connection between events. Aristotle says that history only deals with isolated, particular events, but a good historian can read more general truths into these events, just as a good tragedian can draw general truths out of the stories of particular characters.
Perhaps we would do better to understand Aristotle's distinction as being between fact and fiction. We tell stories to help make sense of a world that at times may seem frighteningly meaningless. There are no beginnings or ends in real life, and the stuff in between is nowhere near as neatly organized as it is in tragedy. The role of the tragedian is to take a certain series of events and to trace a logical sequence between them. The tragic action then shows us that there is some order, some necessity, in the world around us. We learn that certain kinds of behavior, certain choices, lead to certain consequences. Tragedy draws patterns out of a meaningless swirl of experience. The end of the tragedy gives meaning to all that preceded it, as if to say, "these sorts of situations, these sorts of characters, these sorts of decisions, tend to result in this kind of a conclusion."
This causal chain need not be evident; in fact, Aristotle suggests that it is more interesting if it isn't. The best plots have unexpected outcomes, but this does not mean that they take place outside the realm of causality. Rather, unexpected twists make us aware of how poor we are at following the momentum of necessity. To take a modern example, the surprise ending to the movie The Usual Suspects does not make us feel cheated, as if something illogical took place. Rather, it makes us realize how poorly we had understood all the action that had preceded us: it makes us think of the whole movie in a new light.
Aristotle explicitly mentions pity and fear in reference to the logical sequencing and unexpected outcome of tragedy. We see that our character and actions determine our fate with chilling justice and efficiency, but that we are mostly ignorant of the causes of this fate and can never see it coming. We don't need to suffer Oedipus's fate to recognize our own ignorance and vulnerability in the character of Oedipus.