The "marriage plot" refers to a course in 19th century novel the main character, Madeleine (think of the Ludwig Bemelmans' heroine with whom she identified as a child) Hanna takes at Brown. The marriage plot, in which novels end with the suitable marriage of the heroine (a la Jane Austen) or later in which the heroine marries, is unhappy but has to stay married (a la Henry James) is posited as the raison d'être for the novel itself and the question is whether the novel can survive its loss. The theory is that the "marriage plot" was essentially the definition of a novel until social conditions between men and women changed and marriage was no longer the defining moment in the life of a woman. Did the novel languishes without the marriage plot? Madeleine writes a senior thesis on the marriage plot and Jeffrey Eugenides adapts a marriage plot to the late 20th century.
The novel takes place in the late seventies/early eighties when Madeleine goes to Brown. Her background is important to the novel: her father was president of a small college in New Jersey and her mother the epitome of upper middle class East Coast society. Financial and social security was important to both of them. Madeleine is the typical Ivy Leaguer. And while many of Maddy's friends at Brown come from a similar background, the two men who figure as main characters do not. Leonard comes from the west coast and from an arty but fractious family: both parents drink, the father is an antique dealer and eventually leaves his wife to go live with a former client in Europe. Mitchell comes from Detroit, also not from the comfortable middle class. He half identifies with the Greek side of his family (we remember the Greek society in Detroit from Middlesex and there's a bit of that in Mitchell's background--and in Eugenidies').
I'm a generation (or close to a generation) older than the characters (and the author) but was finishing an advanced degree and becoming an asst prof during the time period--in an English department--so I was fascinated by the introduction of deconstruction theories into the literary department conversation. Like most of Madeleine's profs I was schooled in the New Criticism but fascinated by semiotics and structuralism (and deconstruction)--Barthes, Derrida, etc. and with the whole idea of "literary theory" as a discipline (before that there were schools/fashions of literary criticism but nothing so formal as a theory, with philosophical and sociological underpinnings). All three of the main characters study these ideas though Leonard is a biology major and Mitchell in religious studies and they both take it more seriously than does Madeleine who decided eventually on a Victorian lit focus. But it's these ideas that "invalidate" (if that's what you want to call it) the very idea of the marriage plot--or any other plot for that matter. Eugenidies doesn't take it further than that except that clearly he's trying to write a novel with a marriage plot in a time when that plot would seem superannuated. Mitchell has a half-baked theory (discussed with others at Brown) that you can divide people up into first stage (married early, right out of college), married after establishing a career, and others....
The novel is told mostly from Madeleine's point of view, though the two male characters' back stories and aspirations as well as current experiences take the forefront some of the time, Mitchell (clearly a stand-in for the author) gets more center stage than Leonard. Madeleine impulsively invites friend Mitchell home with her for Thanksgiving freshman year. He's in love with her; she considers him a friend. There's an opportunity for a sexual encounter which both in their way back away from. Madeleine meets Leonard in the literary theory class and falls hard. Mitchell (like Knightly in Emma) remains in the background, supporting her when he can and suffering in silence. The perfect marriage plot. And not nearly as different from Emma or The Portrait of a Lady as one might expect, despite Madeleine's grad school plans, her prenup and the possibility of easy divorce, none of which were available to the 19th century heroines.
Maddy falls for Leonard, has a whirlwind affair with him till he breaks it off. She's devastated and Leonard seems to drop off the map--not coming to the class where they met and she doesn't see him anywhere else either. On graduation day, she discovers he's in the psych ward--he hadn't been taking the medicine for his bipolar condition. She misses graduation and rushes to rescue him. Mitchell, still in love with Madeleine, goes off to India as planned with Larry (who deserts him for male lover in Athens) and continues to explore his spirituality, volunteering for Mother Theresa and traveling to Indian shrines. He writes a long letter to Madeleine which he concludes with a plea that she not marry Leonard. She of course does. (This may be the rescue plot rather than the marriage plot--or the marriage as rescue plot.) Her parents are supportive but WASP-conventional when Leonard, experimenting with his meds, goes off the deep end. Mitchell appears out of nowhere to be a support to Madeleine. There is no Jane Austen ending, but the marriage plot seems to have worked quite as well as it did in the 19th century, even with quite a different ending. Or at least that's how I saw it. The novel isn't dead because women have alternatives they didn't have in Jane Austen's time. The marriage plot isn't entirely dead either--it's just different because marriage isn't "final" for the woman either. Whether or not she stays in the marriage, her options are much broader than those in Austen or James