In order for plot to function, it not only needs the basic concepts from the previous chapters, but the following components as well: astonishment, reversal (or peripeteia), recognition, and suffering.
Astonishment refers to a tragedy's ability to inspire 'fear and pity.' Both fear and pity are elicited from an audience when the events come by surprise, but not by chance. The surprise that drives the tragedy must feel like it is part of a grander design.
Reversal is the change by which the main action of the story comes full-circle -- for example, In Oedipus, the messenger who comes to free Oedipus from his fears of his mother produces the opposite effect with his news.
Recognition is the change from ignorance to knowledge, usually involving people coming to understand the identities of one another or discovering whether a person 'has done a thing or not.' The best forms of recognition are linked with a reversal (as in Oedipus) and, in tandem, will produce pity and fear from the audience.
Suffering is a destructive or painful action, which is often the result of a reversal or recognition. Aristotle points out that a 'simple' plot omits a reversal or recognition, but a 'complex plot has one or the other - or both, if it is truly transcendent. All tragedies, however, depend on suffering as part of its attempt to elicit pity and fear from the audience.
Finally, Aristotle points out the structural parts of a tragedy (or 'quantitative' parts, as he calls them). These are the prologue, episode, exode, and choric song.
The prologue is the part of the tragedy which precedes the first undivided utterance of the chorus. The episode is the part of the tragedy between choral songs, and the exode is the first part of a tragedy with no choric song after it.
Three key concepts are introduced in this section - reversal, recognition, and catharsis (though Aristotle refers to the last as 'purgation.') A simple tragedy will have none of these elements (or a perfunctory catharsis), but a complex tragedy will use reversal and recognition to achieve catharsis.
Reversal works in tandem with a story's spine or center to ensure that the hero comes full circle. Oedipus is the best example of a hero who encounters such a reversal -- he hears news that his fears have been allayed, the mystery solved, and then in the course of enjoying this relief and hearing the news, he realizes that it in fact implicates him.
Often, recognition is a tool to achieve this reversal or a byproduct of it -- in this case, Oedipus recognizes the true identity of his father and mother, the nature of his own crimes, and the accuracy of the prophecy. In one swift blow, Oedipus has come full circle and is now the victim of his own search for justice and truth.
The concept of suffering is slightly misleading in that it does not refer simply to a character's endurance of physical and emotional pain. In order to truly produce catharsis - the commingling of fear and pity in an audience - the suffering must be a consequence of reversal or recognition. And indeed, the more surprising the reversal or recognition - as in the case of Oedipus - the more the audience will themselves suffer empathetically, realizing that they too have been ambushed by the causal chain of the plot. Even as 'objective' observers, audience members too are flawed - and thus learn from the tragic hero's fate.
Catharsis, then, is pity for the hero, and fear that his fate could befall us. While pity is the result of any combination of reversal and recognition, fear can only be a product of reversal and recognition crafted into a surprising ending to the plot. And indeed, the absolute pinnacle of tragedy comes when surprise, reversal, recognition, and suffering are united around the core spine of the story in a swift blow to the audience at the end of the third act.