The early stages of The Flies are focused on the setting and mood: what is Argos like? Who are its citizens and how do they live their lives? Because we watch the action through the eyes of Orestes – a foreign visitor– we encounter the town as an outsider.
It seems pretty clear that Orestes wants to commit to something and to take on real responsibility. We also start to suspect that Electra is waiting for her brother to return to Argos, in the hope that he will avenge their father's death.
The plot thickens as the two young siblings have to deal with several external systems of order: that of the gods, that of the King and Queen, and that of "fate." Orestes is confused and unsure of what to do, and Electra flip-flops back and forth between rebellious teen and passive coward.
It's no coincidence that structurally this falls at the center of The Flies. The most important moment for Orestes comes at the end of Act II, Scene i, when he encounters the anguish of personal freedom and decides to act on it rather than flee from it.
Now that the murders are done, Electra and Orestes are on the run and have taken sanctuary at the feet of Apollo's statue. Because The Flies is a philosophical play and not an action thriller, we're not so much looking for a heart-pounding chase scene as we are interested in what's going to happen to Electra. Orestes's resolve is certain, and we don't really doubt his convictions. But Electra is wavering…
This is where Sartre explains much of his philosophy. Orestes is given the opportunity to reflect on his actions and explain his choices to Zeus. This is a chance for Sartre to explain his philosophy to the audience. This is where The Flies deals with ideas like freedom and responsibility, consciousness, and being-for-itself.
Orestes decides where to go from here. Choosing to be "a king without a kingdom," he leaves the freshly liberated citizens of Argos to walk free in the sun.