Aristotle introduces the concepts of peripeteia (reversal of fortune) and anagnorisis (discovery or recognition) in his discussion of simple and complex plots. All plots lead from beginning to end in a probable or necessary sequence of events, but a simple plot does so without peripeteia or anagnorisis while a complex plot may have one or both of these elements. The peripeteia or anagnorisis of a complex plot should themselves be necessary or probable consequences of what came before so that they are a part of the plot and not unnecessary add-ons.

Peripeteia is the reversal from one state of affairs to its opposite. Some element in the plot effects a reversal, so that the hero who thought he was in good shape suddenly finds that all is lost, or vice versa.

Anagnorisis is a change from ignorance to knowledge. This discovery will bring love and happiness to characters who learn of good fortune, and hatred and misery to those who discover unhappy truths. The best kind of anagnorisis accompanies peripeteia. That is, a reversal of fortune effects a discovery or vice versa. For instance, Oedipus' discovery of who his mother is effects a reversal of fortune from proud king to horrible disgrace. Aristotle suggests that anagnorisis is possible by a number of other means as well, but it is most intimately connected to the plot when it accompanies peripeteia. The two together will help to arouse pity and fear and will also help to draw the play to its conclusion.

In addition to peripeteia and anagnorisis, Aristotle defines a third part of the plot—suffering—as actions of destructive or painful nature, such as murders, torture, and woundings.

In Chapter 12, Aristotle discusses the quantitative elements of tragedy—the different parts of the performance. These are the Prologue, Episode, Exode, and a choral portion consisting of Parode and Stasimon. In addition, some tragedies have songs from the stage and a Commos, a lamentation sung by both actor and chorus. The Parode is the first full statement of the chorus; everything that precedes it is Prologue. The Stasimon is a choral song in a certain meter, while action that takes place between choral songs is Episode. Everything that follows the last choral song is Exode.

Peripeteia and anagnorisis are fancy Greek words, but we are all quite familiar with the concepts. Anyone who has watched the eighties television show The A-Team is quite familiar with peripeteia. Every episode, the A-Team thinks they have the bad guys stumped, but then the tables are turned (the first peripeteia), and the team is captured. Of course, the bad guys always lock them up in a warehouse full of welding equipment, and the A-Team builds a big machine, breaks out, and busts the bad guys (the second peripeteia). This example may seem silly, but the point is that peripeteia is not an archaic concept but an incredibly potent literary device that is used effectively in almost every genre at almost every level.

Anagnorisis is similarly ubiquitous. The discovery can be a simple matter of seeing clearly a pattern in events that had seemed obscure before, or it can be a moment of recognition that alters the character's behavior and sense of self. To draw two examples from the movie The Empire Strikes Back, we find a simpler kind of anagnorisis in Luke's discovery that the little green guy is Yoda and the more complex kind in Luke's discovery that he is Darth Vader's son.

The difference between these examples from popular culture and the best of Greek tragedy is the way in which peripeteia and anagnorisis are integrated into the plot of a tragedy. Aristotle insists that these elements not be included unless they are an inevitable part of the necessary or probable sequence of events that leads from beginning to end. The reversals in each episode of The A-Team are hardly necessitated by events; they usually seem forced and improbable. They are simply cheap devices to keep the audience guessing.

The unity of plot in Greek tragedy is meant to clarify a pattern in events that helps us to understand the consequences of our thoughts and actions. Peripeteia and anagnorisis essentially help us to recognize why these patterns are not immediately evident to anyone with a little life experience. Life is not a simple progress from A to B, but it involves reversals that upset our best laid plans. Further, we are far from aware of the many factors—both in ourselves and in the world around us—that determine our fate, and we often only learn of some important factor through a moment of belated recognition. A tragedy that includes peripeteia and anagnorisis allows us to see the inevitability of certain fates and also makes us understand why we are so often unable to perceive these fates.

Chapter 12 is an odd intrusion that interrupts Aristotle's discussion of plot. There is some question as to whether this chapter is in fact Aristotle's, or at least whether he intended it to be inserted into the discussion where it is. It seems oddly limiting in a way quite different from the discussion of the unity of plot. The call for a tightly structured plot may be applied to some extent to modern tragedy, but the requirement that there be a certain number of choral songs hardly seems to be a necessary element. Again, we should recall that Aristotle is primarily an observer and only sometimes a legislator. In discussing the quantitative parts of a tragedy, he may be simply remarking on what he has observed.

At the very least, this chapter helps us understand how the choral songs and speeches by actors are meant to frame one another. We have a spoken Prologue and Exode that frame all the choral songs and the Episodes that are inserted between choral songs. We might think of the choral songs as being like the refrain in a pop song, and the spoken bits as being like the verses. The spoken bits advance the action and deal with the particulars of the play, while the choral songs frame the action and discuss the overall themes of the play.