Set just after the Civil War, the play tells the story of a proud New England family whose history echoes that of the House of Atreus in Greek mythology. As the title suggests, Mourning Becomes Electra loosely reworks the first great surviving trilogy of plays, The Oresteia by Aeschylus.
The first part of the trilogy, Homecoming, portrays the return of the victorious Brigadier-General Ezra Mannon. (Get it? Ezra Mannon? Agamemnon?) His wife Christine hates him, though, and has fallen in love with a ship captain named Adam Brant, whom O'Neill describes as having "a romantic Byronic appearance." Needless to say, things do not go well for Ezra Mannon.
Christine is in constant conflict with her daughter Lavinia, the Electra figure of the trilogy. Even in Homecoming Lavinia wears a "plain black dress," but in the next play, The Hunted, the whole household has gone into mourning. Lavinia's brother Orin returns, and she and Christine battle each other trying to win him over. Lavinia proves the truth to Orin about their mother and Adam Brandt, and things don't go well for Brandt, or for Christine, either.
The third part of the trilogy, The Haunted, departs the most from Aeschylus's version of the story. While the ancient Greek writer ends the myth with forgiveness and redemption, O'Neill portrays an America where such things are impossible. Christian theologians identify pride as the deadliest of all sins, in part because sinners indulging in pride can think so much of their transgressions as to believe forgiveness of them is impossible. They cannot comprehend of a force greater than their own wickedness.
This is where O'Neill's play seems to intersect with American Exceptionalism. Traditionally, we think of American Exceptionalism being about America's inherent goodness, and how no country on earth could ever be better. The flip side, however, is a pathological focus on the wickedness of America. No other country has ever had slavery! No other country has ever fought such a bloody civil war! No other country has been as treacherous and deceitful as the United States of America! All of this is hogwash, of course. In terms of evil, Hitler's Germany, Stalin's Russia, and Mao's China can all give us a run for our money.
But the characters in Mourning Becomes Electra are so focused on their own crimes that they can't even imagine redemption. While Orestes is given mercy in the original, O'Neill's Orin denies himself peace, in spite of the saintly Hazel seemingly ready to forgive anything out of her love for him. Lavinia is beloved by Peter, who if he knew the truth, would probably marry her anyway and be happy about it. "Love isn't permitted to me," Lavinia says. And why not? Lavinia explicitly states that she will not ask God or anyone else for forgiveness. Instead, she says she will forgive herself.
That is something she cannot do. Only by going beyond the self can we ever find forgiveness, and to say that no one else can forgive us is a tremendous act of pride, the sin of the entire Mannon family.