Beginning, Middle and End
The concept of unity guides nearly all of Aristotle's proclamations about narrative structure. He explains that you can test the unity of a work by removing a part or reordering parts. If the plot's meaning and sense survive, the work is not unified; it has superfluous parts. In a good plot, every occurrence results from the previous occurrence. Every portion is crucial to the plot's development. Incompleteness is no better than excess, but if a plot has a beginning, middle and end, it is whole. This sounds like a relatively low standard, but Aristotle has specific definitions of beginning and end. They must be at opposite ends of the cause-and-effect chain that forms the substance of the play. A beginning has no determining cause; an ending has no effect.
Complication and Denouement
Aristotle also divides plot into complication, turning point and denouement. The turning point is a change of fortune, either from bad to good or from good to bad. Naturally, the latter is appropriate in a tragedy, but Aristotle adds that a tragic plot has the greatest emotional impact if it takes place within a day. Complication is everything that leads up to the turning point, and denouement, or unraveling, is everything that follows it. Based on his standard of causal relationships, Aristotle's theory of poetry prohibits using "deus ex machina," divine intervention, within the action of the play. It may precede the play -- for example, a character may recount a prophecy -- but otherwise it disrupts plot unity.
Simple and Complex Plots
All plots entail a change of fortune, but Aristotle argues that the best plots are complex, meaning that their changes arise from reversal of the situation, recognition or both. In reversal, the planned and intended occurrence fails to happen, and the opposite happens instead. Aristotle cites the tragedy of Oedipus as an example. In it, the messenger who intends to reassure Oedipus that he avoided his prophesied fate ends up horrifying Oedipus by revealing the reverse. This moment also works as an example of recognition: Oedipus has already killed his father and married his mother when the messenger arrives; his fortune changes when he recognizes this situation, in what Book 11 of "Poetics" calls the "change from ignorance to knowledge."
Epic and Tragedy
Aristotle notes that epic poetry and tragedy have much in common, but the fact that tragedy is staged makes for some differences in their narrative structures. Epic has a wider scope. It extends beyond a single day and depicts multiple simultaneous events. Also, audiences enjoy repetition in epic poetry but find it boring in Tragedy. Although epic poetry can contain longer scenes, and more of them, Aristotle asserts that the plot at its heart ought to be easily summarized. He illustrates his point with a 67-word summary of Homer's lengthy "Odyssey," omitting the hero's various entanglements to emphasize his initial misfortune and ultimate triumph. "This is the essence of the plot," Aristotle explains in Book 17; "the rest is episode."