The play starts out on a rather even keel for the characters. They've lived this way for twelve years in relative peace, with Prospero teaching Miranda – yet there's room for improvement under an auspicious star.
The peace that Miranda and Prospero have known is suddenly shattered by the presence of all these strangers. Of course, what's funny about The Tempest is that Prospero essentially willed the villains to him, so he's kind of created his own emergency – on purpose. At least with them around, he has a conflict to resolve. Faced with all these bad guys, Prospero can either wreak havoc upon them with his vengeance, or forgive and maybe get his dukedom back. Also, he'd like his daughter to fall in love with the Prince of Naples, which he hopes is in line with her desires, though he doesn't really know if it will work. Despite the fact that Prospero seems to be in control of the action, we're unsure of how everything will turn out. Kind of.
Though Prospero has called the conflict to him, putting all these wicked people on the island together is bound to create unanticipated problems in a seemingly controlled situation. Sebastian agrees to murder the King, aided by Antonio. Elsewhere, Caliban is plotting to murder Prospero. While the little murder plots don't really deeply impact Prospero or his goals, they're still trifles he has to deal with. Prospero knows that all of this is happening. Regardless, Prospero is busy creating complications for Ferdinand and Miranda by pretending he hates Ferdinand, so the young couple will take their love seriously having struggled for it. He's plagued by complications, but he's not above adding to them for his own ends.
The harpy chides all the traitors. Alonso in particular is singled out by the monster for the great wrong that was done to Prospero, claiming this past treachery is the source of the sea's anger at Alonso, and for this, the sea claimed his son. Up to this point, Prospero hasn't made any action to reveal himself or his power to the shipwrecked folks, and they've all been wandering around aimlessly. With the harpy incident, we get the satisfaction of knowing that not only does Prospero plan to deal with the villains, but he's not above messing with their heads a little bit, too. For the first time, we know justice will be served.
The three traitors are unraveled by the harpy incident. The King is distraught at the confirmation that he does indeed suffer for what he's done to Prospero. Gonzalo is afraid of what the King might do in his crazed state. Antonio and Sebastian, murderous even when not crazed, have now vowed to fight the spirits one by one. We're on the edge of our seats (or pages?) trying to figure out how each of the villains will respond to the harpy warning, and what will come of their reaction. Everyone is tense, with the saner group of the party in pursuit of the other three, hoping to stop them from doing anything drastic. Meanwhile, a drunk and power-lusty Stefano approaches Prospero's cell, with a coolly calculating and enraged Caliban by his side, all planning Prospero's bloody murder. Will Caliban succeed? Will Prospero die?
Prospero inquires after where the three traitors Alonso, Antonio, and Sebastian are. Ariel informs his master that the men are all tied up in some strange states. Although Ariel is not human, he thinks that anyone who looked at them might be moved to pity the three traitors. Prospero is moved by Ariel's gracious thought and decides to act on his virtue rather than his vengeance. Once Prospero has decided to treat the villains gently, he relaxes any tension we felt. We know the falling action of the play has begun, with resolution to follow. Also, hearing of Caliban's planned treachery, Prospero gets Ariel to lay out his fine robes, which break Stefano and Trinculo's concentration on the deed against Prospero. Caliban, though enraged, sees the men for the distracted fools they are, and all are sent forth running by a comedic pack of bloodthirsty hounds. Caliban's renunciation of the real bad guys also adds to the falling action; he's no longer a dangerous rebel.
With all the spells broken, Prospero gathers everyone and gets to say his piece. Alonso, having survived an almost-attempted assassination, gives Prospero back the dukedom of Milan, which neutralizes the threat of Antonio and Sebastian. We find out from the King that Stefano is merely his harmless, drunken butler (which we think is some assurance that he wouldn't really have tried to cut out Prospero's windpipe), and Caliban repents for his foolishness, wishing to be once again in Prospero's good graces. Most importantly, the rift between Naples and Milan will be sealed up in the marriage of Miranda and Ferdinand. Everything is declared finished, which is the defining stuff of conclusions.