The principal theme of The Seven Against Thebes is the role of fate in human affairs and more particularly the malign role of the Fates in dooming Oedipus and his descendants. As is generally the case in the extant Greek tragedies, the theme is demonstrated by the action of the play, and directly articulated by the chorus as they comment on the events.
The action of the play occurs between the two better-known stories of Oedipus and Antigone. Eteocles, son of Oedipus, is the king of Thebes, and defends the city against an army which includes his brother, Polyneices. Eteocles decides that, of all the seven gates of Thebes, he will defend the one which he knows Polyneices will be attacking. He does this even though he is well aware of the curse on the House of Oedipus and the likelihood of one or both brothers being killed. He decides that he will allow fate to decide the issue. In the event, both brothers are killed.
The role of fate extends beyond death and beyond the end of the play, since the burial of Polyneices provides the subject for the story of Antigone. In this respect, even the names of the brothers reveal their fates, since "Eteocles" means "rightful glory" while "Polyneices" means "many troubles."
The quotations which comment on the role of fate come mainly from the chorus, as in the lines:
The sign of Ruin standeth at the gate,
There, where they strove with Fate-
And the ill power beheld the brothers' fall,
And triumphed over all!
At the end of the play, the chorus addresses the Fates directly:
Exulting Fates, who waste the line
And whelm the house of Oedipus!
Fiends, who have slain, in wrath condign,
The father and the children thus!