Aeschylus is one of the earliest playwrights whose work survives, and it is clear from a play like Prometheus Bound that drama was in its basic stages. In addition to the chorus there are only two speaking characters on stage at any one time, and much of the play consists of fairly lengthy monologues describing the sufferings of either Prometheus or Io. Not much action takes place during the play itself, though a good amount of action is narrated. The only exception to the lack of action occurs at the beginning of the play, and slightly at the end. The play opens with Hephaestus being ordered by Strength to nail Prometheus to the ground for his Zeus ordered punishment. Then the play ends with the ground opening under Prometheus to swallow him up.
Between these two events, the play focuses on two parallel stories of suffering under Zeus' will--the suffering of Prometheus bound to the rock, and the suffering of Io driven mad and forced to wander by Hera in revenge for Zeus' love. Aeschylus seems interested in the divergence between Zeus' will and the force of a law based on justice. The play is a meditation on the shift from barbarian societies where violence ruled and strength was the only law to an emerging Greek civilization with codes of laws based on ethics and justice.
After Hephaestus reluctantly binds Prometheus under Strength's orders, Prometheus is joined by the Chorus of Oceanus' daughters who sympathize with him and try to find out what Prometheus' crime was and how he might be released from his torment. They are shortly joined by Oceanus who laments Prometheus' plight and proposes to go and convince Zeus to free his brother god. After Oceanus leaves, Prometheus and the Chorus are joined by Io--a girl tormented by Hera with madness, horns, gadflies, and wandering. The Chorus asks Prometheus to explain Io's story, but he insists that she should tell the story. She relates how Zeus fell in love with her and tried to seduce her, at which point Hera began to torment her. She asks Prometheus to tell her future, and Prometheus obliges, narrating a series of wanderings across the globe. Finally Io leaves under a new gadfly attack, and Hermes comes in with a message and threat from Zeus. Prometheus has prophesied that Zeus' reign will end when he makes a bad marriage, and that only Prometheus can save Zeus' kingdom. Zeus, via Hermes, demands to know the details of this marriage and how to prevent it. Prometheus says that he will not tell Zeus how to prevent the overthrow until Zeus rescinds his judgment of Prometheus. In vengeance for the resistance, Zeus sinks Prometheus into the earth and dooms him to be raised eventually and to have his liver eaten by an eagle everyday, only to have it grow back nightly and be eaten again.