There is really very little plot in The Tempest. There is the love story, and then there is the story of two younger brothers who covet their older brothers' titles and possessions. And finally, there is the story of Caliban's plot to murder Prospero. But none of these plots are given much attention or substance; instead, the play is about the complexities of human nature and about reminding the audience that the division between happiness and tragedy is always fragile and must be carefully maintained.
Although The Tempest ends with the promise of a wedding, it could just as easily have ended with tragedy. In this play, there are two murder plots and a betrayal to resolve. In a tragedy, these might have ended with the stage awash in blood, as in Hamlet, but in The Tempest, Prospero's careful manipulation of all the characters and their plans also controls the direction of the action. Prospero's avoidance of tragedy reveals his character's decency and contradicts some critics' arguments that he is an amoral demigod exploiting the natural inhabitants of this island.
The Tempest is unique in its adherence to the three unities. In his Poetics, Aristotle argued that unity of action was essential for dramatic structure. This meant that a dramatic work should have a clear beginning, middle, and end. The unity of time is derived from Aristotle's argument that all the action should occur within one revolution of the sun — one day. The unity of place developed later and is a Renaissance idea, which held that the location of the play should be limited to one place. These unities added verisimilitude to the work and made it easier for the audience to believe the events unfolding on stage.
Shakespeare rarely used the three unities, but he uses them in this play, something he has only done in one other play, The Comedy of Errors. All the events occur on the island and within one brief three-hour period. Shakespeare needed the three unities, especially that of time, to counter the incredulity of the magic and to add coherence to the plot.
The Tempest, although it is one of Shakespeare's shortest plays, still maintains the integrity of the five-act structure. In fact, most Elizabethan theatre adheres to the five-act structure, which corresponds to divisions in the action. The first act is the Exposition, in which the playwright sets forth the problem and introduces the main characters. In The Tempest, the first act establishes the nature of Antonio's betrayal of Prospero, and it explains how Prospero and Miranda came to live on the island. This first act also opens with a violent storm, which establishes the extent of Prospero's power. Most of the play's remaining characters also make an appearance in this act.
The second act is the Complication, in which the entanglement or conflict is developed. In The Tempest, the conspiracy to murder Alonso is developed, which establishes that Antonio is still an unsavory character. In addition, the audience learns more about Caliban, and Stefano and Trinculo appear, allowing the groundwork for a second conspiracy to be formed.
The third act is the Climax; and as the name suggests, this is when the action takes a turning point and the crisis occurs. In a romance, this is the point at which the young lovers assert their love, although there may be complications. It is important that the way to love not be too easy, and so in The Tempest, Prospero has forbidden contact between Miranda and Ferdinand, although the audience knows this is only a pretense. In this act, the conspiracy to murder Prospero is developed, although the audience knows that Ariel is listening, and so there is no real danger. And finally, the essential climactic moment occurs in this act when Prospero confronts his enemies at the ghostly banquet.
The fourth act is called the Falling Action, which signals the beginning of the play's resolution. In this act, the romance between Ferdinand and Miranda is acknowledged and celebrated with a masque, and Prospero deals with the conspiracy to murder him by punishing Caliban, Stefano, and Trinculo.
The fifth act is called the Catastrophe, wherein the conclusion occurs. As the name suggests, this act brings closure to the play, a resolution to the conflict, and the plans for a wedding. As the play draws to a close, Prospero is victorious over his enemies, Ferdinand is reunited with his father, Antonio and Sebastian are vanquished, and Caliban regrets his plotting.