To Aristotle, the plot is the most important element of the tragedy, and the best tragic plot is to be single and complex. Also he said “in the end of the play, the audience should experience the feeling of fear and pity”
The plot must have two major parts. First, set up a problem. Then, set in motion the denouement (unraveling) that resolves the issue. And to Aristotle, the most powerful part of the plot was a sudden turn in the fortune of the main character, the turning point of the play might be the discovery of the true identity of the hero or it might be the revelation of a long forgotten incident from the past.
Aristotle also said "the nature of tragedy was to arouse pity and fear”. And to arouse these feeling, the playwright needed a tragic hero. And the tragic hero must have four important qualities. The first one, the character must be good, and must be elicit sympathy from the audience. The second quality is the character must have appropriate characteristics. Thirdly, the character must be true to life. This means the character must be realistic and relevant to the real life. The fourth point is the character must act according to a consistent psychology, so the character shouldn’t being a strange person.
The ideal tragic hero, according to Aristotle, the tragic hero commits a mistake because of some sort of ignorance and he doesn’t belief that what he doing is wrong. Thus, the hero downfall must be caused by himself not by any other person. Then we come at what Aristotle calls catastrophe or reversal of fortune (form high status to low status). After the catastrophe occurs, there is a stage of anagnorisis, which involves the tragic hero understanding what has happened and he submits to his fate.
If the play has happened as it should, then the audience should experience emotions of pity and fears. The purpose of tragedy was to bring about a cleansing or catharsis of these feeling.