Classical Unities

'The classical unities are rules for drama derived from a passage in Aristotle's Poetics. In their neoclassical form they are as follows:

1. unity of action: a play should have one action that it follows, with minimal subplots.
2. unity of time: the action in a play should occur over a period of no more than 24 hours.
3. unity of place: a play should exist in a single physical space and should not attempt to compress geography, nor should the stage represent more than one place.'

From wikipedia - https://en.wikipedia.org/wiki/Classical_unities

The Tempest does follow the first rule Unity of Action, the main plot line being Prospero getting his 'revenge' on Antonio and the other Dukes and regain his rightful title. Although it could be argued that the sub-plot of Caliban, Trinculo and Stephano is more than an minimal subplot since it  interrupts the Masque for Miranda and Ferdinand when Prospero remembers he must deal with these three. On the other hand this could show how it is an almost insignificant story since Prospero, the main character and who will be affected by this subplot, almost forgets about it, alluding to how unimportant it is for audience and play.

Following the rule of Unity of Time the Tempest is set over a less than a 24hr period, we learn this when Prospero tells Ariel of his plan. 

In Act 1, Scene 2 Prospero tells Ariel 'At least two glasses. The time 'twixt six and now must by us both be spent most preciously'.

'twixt six and now' tells the audience that now being 2pm (two glasses) and six is the time frame of the plan. The real time of day would often correspond to this, the real life time and play time being almost parallel. 

The Tempest stage does cover many different places, although they all are on the same island, the play can be seen to not follow the rule of Unity of Place that the stage should represent only one place. This is because the action takes place in many different locations on the island, therefore not all in a 'single physical space'. Although they still all do take place on the same island, apart from the first scene which takes place on the boat in the storm.

It can be argued that Shakespeare did consciously choose to abide to the rules of the classical unities in The Tempest since unlike his other plays it does follow, to some extent, all three rules. Some critics believe this is because Shakespeare thought this to be his last play and wanted to show that he could adhere to this classical style when he wanted to, presenting his ability as a playwright. On the other hand because none of his other plays did follow this style it could be a coincidence, Shakespeare 'simply wished to concentrate the audiences attention on the theme of reconciliation which feature prominently in the play'(YorkNotes) and in doing so found himself adhering to the classical unities.