Act 1 sets the scene for the rest of the play, establishing characters and themes. The Titan Prometheus is being punished by the gods for stealing fire and giving it to mortals. It's a truly terrible punishment; Prometheus is tied to a rock where his liver will be eaten by an eagle each day. At night the liver will grow back only to be eaten by an eagle once more the following day; and so on for all eternity.

The first act establishes the profound hatred that Prometheus feels toward Jupiter (the Roman equivalent of Zeus) for putting him through this terrible ordeal. Shelley also introduces us to the feminine aspect of Prometheus Unbound, something that will come to play a major part in the rest of the drama. The Earth comforts Prometheus, acting as a mother to him as with all those afflicted by Jupiter's tyranny. And then there's Asia, the love of Prometheus, with whom he expresses his desire to reunite.

Throughout the first act Prometheus remains defiant. He wants to recall the curse he put upon Jupiter, but the Earth dare not repeat it. So Prometheus summons up the Phantasm of Jupiter to utter his fateful words:

I curse thee! let a sufferer's curse / Clasp thee, his torturer, like remorse; 'Till thine Infinity shall be / A robe of envenomed agony; And thine Omnipotence a crown of pain, To cling like burning gold round thy dissolving brain.

But Prometheus then appears to recant, saying "I wish no living thing to suffer pain." The Earth mistakes this as a sign that Prometheus has finally been vanquished by Jupiter. However, that's not the case. Prometheus is simply asserting his belief that mortals should also defy the gods and no longer be subjected to their tyranny.