The first part of O'Neill's trilogy is called Homecoming, and it begins with the end—the end of the Civil War, that is.
Lavinia Mannon is crazy excited for the return of her father, Brigadier General Ezra Mannon, and her brother, Orin Mannon, both of whom have been fighting for the Union during the war. Seth Beckwith, an old groundskeeper who's worked for the Mannons for decades, tells Lavinia that he's worried that Captain Adam Brant—a single sea captain who's been spending a little too much quality time with Lavinia's mom Christine—may actually be the illegitimate child of Lavinia's long-lost great uncle, who was disowned for having an affair with a nursemaid and getting her pregnant.
Not long after that, Lavinia sees Brant coming up the driveway. Lavinia's nasty insinuations about his low-class mother are so insulting that eventually Brant admits to being the illegitimate son of Lavinia's (not-so) great uncle.
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Lavinia storms off and starts an argument with her mother, Christine, who admits that she's sleeping with Brant and knows all about his past. Lavinia threatens to tell Ezra everything if Christine doesn't stop messing around with Brant. Then she storms off again. (Is it just us, or is there a pattern, here?)
Desperate, Christine manipulates Brant into helping her poison her husband. The night that Ezra returns home from the war, Christine ends up murdering him by swapping out his heart medicine for tabs of arsenic. Ezra dies, but not before he manages to clue Lavinia in to the fact that Christine killed him.
It's two days after Ezra Mannon's murder at the start of The Hunted, the second play in O'Neill's trilogy. Lavinia's still furious, and Christine's terrified of what she might do. Lavinia manages to convince Orin—who's way, way too devoted to his mother (seriously, it's creepy)—that Christine's having an affair with Brant and that both of them conspired to murder their father.
A couple of days and some heated arguments later, Orin and Lavinia make their way down to the harbor where Brant's ship is docked. Orin, in what looks a lot like a jealous rage, puts a bullet into Brant. Lavinia and Orin steal a bunch of stuff from the ship and dump it into the water to make it look like a robbery. Then they go home and confront Christine, letting her know that they just killed her lover. Apparently overcome with grief, Christine goes inside the Mannon home and kills herself.
On that cheerful note, we head off into The Haunted, the third play of the trilogy. Orin and Lavinia have just gotten back from a long vacation in the South Seas, and they run into their two friends (and soon-to-be more than friends) Peter and Hazel. Lavinia looks healthy, hot, and happy; Peter insists she looks a lot like her dead mother. Orin, on the other hand, is a hot mess of guilt and borderline insanity.
Lavinia tries to manage her brother's moods, and is super worried he'll tell somebody they murdered Brant. Everybody tries to make the best of it while Orin seems to get weirder and weirder. Consumed by guilt, and wanting to rejoin mommy, he kills himself, even though he was supposed to marry Hazel. Lavinia, who was supposed to marry Peter, lies about having an affair so he'll dump her, which he does.
Lavinia decides that she's doomed to live out her days in the Mannon home, surrounded by the ghosts of her dead relatives, as punishment for what her family's done. Guilt's going to keep her company for the rest of her life, and the play ends as Lavinia retreats into the house forever.