Aristotle characterises tragedy as possessed of a unity of action, time and place. Aristotle described unity of action as follows: “tragedy is an imitation of an action that is complete, and whole, and of a certain magnitude.” In other words, the main action of tragedy should be something serious (murder, ritual pollution, the rise and fall of a ruler) or non-trivial. The tragedy, although it may start in medias res, and report its backstory, as it were, should conclude with a resolution. This meant that a tragedy should have a single major plot, in which a protagonist underwent a reversal of fortune. Unlike epic, the scope of tragedy should be restricted to a single plot line. The action of a tragedy should take place in a single 24 hour period (although earlier actions may be reported) and should be set in a single place.
In insisting upon the unity of plot, Aristotle makes it clear that he does not mean that it is enough to focus the plot on the life of one individual. Our lives consist of all sorts of disconnected episodes, and the story of a man's life would rarely have the completeness necessary for a unified plot. Rather, the poet must select some series of events from a character's life—as Homer does in the Odyssey —and craft them into a coherent whole. Any part of a story that could be added or removed without any great effect on the rest of the story is superfluous and takes away from the unity of the piece.