Aristotle devotes great attention to the nature, structure and basic elements of the ideal tragic plot. Tragedy is the depiction of action consisting of incidents and events. Plot is the arrangement of these incident and events. It contains the kernel of the action. Aristotle says that plot is the first principle, the soul of tragedy. He lists six formative elements of a tragedy – Plot, character, thought, melody, diction, spectacle and gives the first place to plot.
The Greek word for 'poet' means a 'maker', and the poet is a 'maker', not because he makes verses but he makes plots. Aristotle differentiates between 'story' and 'plot'. The poet need not make his story. Stories from history, mythology, or legend are to be preferred, for they are familiar and understandable. Having chosen or invented the story, it must be put to artistic selection and order. The incidents chosen must be 'serious', and not 'trivial', as tragedy is an imitation of a serious action that arouse pity and fear.
Aristotle says that the tragic plot must be a complete whole. It must have a beginning, a middle and an end. It must have a beginning, i.e. it must not flow out of some prior situation. The beginning must be clear and intelligible. It must not provoke to ask 'why' and 'how'. A middle is consequent upon a situation gone before. The middle is followed logically by the end. And end is consequent upon a given situation, but is not followed by any further incident. Thus artistic wholeness implies logical link-up of the various incidents, events and situations that form the plot.
The plot must have a certain magnitude or 'length'. 'Magnitude' here means 'size'. It should be neither too small nor too large. It should be long enough to allow the process of change from happiness to misery but not too long to be forgotten before the end. If it is too small, its different parts will not be clearly distinguishable from each other. Magnitude also implies order and proportion and they depend upon the magnitude. The different parts must be properly related to each other and to the whole. Thus magnitude implies that the plot must have order, logic symmetry and perspicuity.
Aristotle considers the tragic plot to be an organic whole, and also having organic unity in its action. An action is a change from happiness to misery or vice versa and tragedy must depict one such action. The incidents impart variety and unity results by arranging the incidents so that they all tend to the same catastrophe. There might be episodes for they impart variety and lengthen the plot but they must be properly combined with the main action following each other inevitably. It must not be possible to remove or to invert them without injuring the plot. Otherwise, episodic plots are the worst of all.
'Organic unity' cannot be provided only by the presence of the tragic hero, for many incidents in hero's life cannot be brought into relation with the rest. So there should be proper shifting and ordering of material.
Aristotle joins organic unity of plot with probability and necessity. The plot is not tied to what has actually happened but it deals with what may probably or necessarily happen. Probability and necessity imply that there should be no unrelated events and incidents. Words and actions must be in character. Thus probability and necessity imply unity and order and are vital for artistic unity and wholeness.
'Probability' implies that the tragic action must be convincing. If the poet deals with something improbable, he must make it convincing and credible. He dramatist must procure, "willing suspension of disbelief". Thus a convincing impossibility is to be preferred to an unconvincing possibility.
Aristotle rules out plurality of action. He emphasizes the Unity of Action but has little to say about the Unity of Time and the Unity of Place. About the Unity of Time he merely says that tragedy should confine itself to a single revolution of the sun. As regards the Unity of Place, Aristotle said that epic can narrate a number of actions going on all together in different parts, while in a drama simultaneous actions cannot be represented, for the stage is one part and not several parts or places.
Tragedy is an imitation of a 'serious action' which arouses pity and fear. 'Serious' means important, weighty. The plot of a tragedy essentially deals with great moral issues. Tragedy is a tale of suffering with an unhappy ending. This means that the plot of a tragedy must be a fatal one. Aristotle rules out fortunate plots for tragedy, for such plot does not arouse tragic emotions. A tragic plot must show the hero passing from happiness to misery and not from misery to happiness. The suffering of the hero may be caused by an enemy or a stranger but it would be most piteous when it is by chance caused by friends and relatives who are his well-wishers.
According to Aristotle, Tragic plots may be of three kinds, (a) Simple, (b) Complex and (c) Plots based on or depicting incidents of suffering. A Simple plot is without any Peripety and Anagnorisis but the action moves forward uniformly without any violent or sudden change. Aristotle prefers Complex plots. It must have Peripeteia, i.e. "reversal of intention" and Anagnorisis, i.e. "recognition of truth". While Peripeteia is ignorance of truth, Anagnorisis is the insight of truth forced upon the hero by some signs or chance or by the logic events. In ideal plot Anagnorisis follows or coincides with Peripeteia.
'Recognition' in the sense is closely akin to reversal. Recognition and reversal can be caused by separate incidents. Often it is difficult to separate the two. Complex plots are the best, for recognition and reversal add the element of surprise and "the pitiable and fearful incidents are made more so by the shock of surprise".
As regards the third kind of plot, Aristotle rates it very low. It derives its effect from the depiction of torture, murder, maiming, death etc. and tragic effect must be created naturally and not with artificial and theatrical aids. Such plots indicate a deficiency in the art of the poet.
In making plots, the poets should make their denouements, effective and successful. Unraveling of the plot should be done naturally and logically, and not by arbitrary devices, like chance or supernatural devices. Aristotle does not consider Poetic Justice necessary for Tragedy. He rules out plots with a double end i.e. plots in which there is happiness for one, and misery for others. Such plots weaken the tragic effect. It is more proper to Comedy. Thus Aristotle is against Tragi-comedy.