Aristotle's Poetics in 5 Easy Steps. Step 4: Beginning. Middle. (Twist). End.

Step 4:  Beginning.  Middle. (Twist).  End.   They're not arbitrary points in the story.   Remember, your story describes a single human action (the tragedy), made up of multiple characters influencing and creating that one action.  Human actions begin in certain ways, end in certain ways, and are attained (middle) in certain ways.  Human actions don't happen by virtue of gravitational pull, or the laws of thermodynamics.  They happen because people desire and want things.  Your beginning must express the situation the character is in and their desire for a certain end -- be it an object, a state of being, a solution to a problem, whatever.  That want is held by a person (or group of people) and impacts other people (the other characters).   The desires and wants of other characters impacts the main character's means (middle) to the end.  That the Tragic Hero does not know everything (the 'hamartia') is the basis of the twist ending -- as a new reality and set of circumstances is revealed to him or her, and the tragic reality is introduced.  And to that point, the middle of the story is precisely where those two different realities (the one perceived by the main characters at the beginning, and the true reality described in the twist) collide.  The middle is often described in terms of conflict and complications.  Those conflicts and complications are the clash of those two realities.  The middle thus "builds" to a crescendo of the revelation of the true reality and the reversal of fortune for the main character (the twist).

DO NOT RESTRICT YOUR STORY WITH THE SO-CALLED "CLASSICAL UNITIES"

One of the worst interpretations of The Poetics involves the so-called "classical unities".  These "unities" state that the tragedy must be one action occuring in one place at one time.   This interpretation is derived by conflating action with motion.  It states, thus, that since all motion is understood by referencing absolute space and absolute time, all action can therefore only be understood in the same way.  Thus, the tragedy must occur in absolute space (one location) and absolute time (one day).  This conflation of action and motion is the basis of the bad interpretation.  Human action, however, is not the same as bodies in motion.  Human action involves intention, a goal, morality and character, unforeseen consequences, etc.  Bodies in motion, on the other hand, involve vectors through timespace (or, in Newtonian terms, a body moving though absolute time and absolute space).   The so-called "classical unities" is the horrible attempt to construct an "absolute time and space" through which the tragedy occurs.   The good news is that the so-called classical unities have largely been disregarded in practice.  The bad news is that for whatever reason, high school English teachers (and some college professors) croon over them like.. well, whatever it is they tend to croon about.  Ignore their crooning.  The classical unities don't exist, have never existed, and should not be used as "rules" of drama.  If a tragedy needs to occur in a darkened room with a bunch of people sitting in it, then by all means, have the tragedy occur that way.  But if your tragedy takes place in a variety of places, then so be it. The only "unity" you need to be concerned about is the unity of plot and the unity of character.  That's it.