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.