Poetics: Book XXIII
In Book XXIII, Aristotle turns to the subject of the epic. He views the epic in terms of the tragedy, stating that the epic should be constructed with an eye to the drama (we assume he means such factors as pacing and dialogue should be given careful attention) and that, like the tragedy, it should concern a single, unified action. Aristotle himself uses metaphor to emphasize the need for coherence in a plot: the epic (and the tragedy as well) should be like a living organism in which all parts have their special and indispensable function.
Homer is Aristotle's favored example in his discussion of the epic. Rather than trying to represent the entire complex phenomenon of the Trojan War, Homer focuses on a single event in each of his epics, bringing in other events as episodes to add drama and diversity without distracting from the central story. Homer's epics are for this reason most like tragedies: their stories are focused and unified wholes, not simply sequences of episodes.