This is the second play in the Oresteian trilogy and is basically the same plot as the over versions of the Electra that were also composed by Sophocles and Euripides. However it is very noticeable that it is the second part because this play goes straight into the action with a discussion between Orestes and Pylaides as they stand before Agamemnon's grave. Many of the other plays seem to be self contained, but we understand that Aeschylus wrote mostly trilogies (and it is quite clear that the other playwrights would write their plays as single units). In a way, an Aeschylan tragedy is more like a play in three acts as opposed to three separate plays. They work the way most trilogies work (or are supposed to work) in that they function as a whole as opposed to three separate parts.
In ancient times going to the theatre was a whole day affair, with four plays being performed one after the other (with breaks in between). Normally there would be three tragedies and what is termed as a satyr play: not strictly a comedy but more of a light hearted break from the intensities of the tragedies. However it appears that the trilogy as a form of play was really a more earlier example than the later ones that we have. However, with the single unity plays it is difficult (for me at least) to see how the other plays would function, whether they would be connected or distinct, though I am leaning towards distinct units.
The Libation Bearers (which is the English title of this play) also differs in that there is no strict unity of place, though it appears that the action occurs in a single day (being the unity of time). The location changes four times in this play, beginning at the grave, them moving to the city, and then moving into the palace where the murders take place. The play begins with Orestes' arrival and with Electra quickly identifying him (this takes longer in the other versions) and the Orestes and Pylaides sneaking into the palace to avenge his father's death. The play then finishes with a horde of furies descending upon Orestes and setting the scene for the final play in the trilogy.
It appears that the story is quite a popular one, though it probably has a lot to do with the moral dilemma that the play centres around. It was the duty of the son, in Greek culture, to avenge a father's murder and vengeance was always an eye for an eye, meaning that the killer must die. However this was not a simply trial by jury, but bloody vengeance, and in a way it was acceptable. What was not acceptable was matricide (the murder of a mother) and thus comes the moral dilemma. Agamemnon was murdered, and it is Orestes' duty to avenge his death. However Agamemnon was murdered by his wife, and it was repulsive for a child to slay his mother but he had to avenge his father's death. As such, when the deed was done, the furies descend upon him to drive him out of Argos and he also had to seek cleansing and purification.
Thus the stage is set for the final chapter in this drama, however I will not go any further into this as I shall leave the final instalment for the Eumenides.