(I’ll break my own rule here and use the more faithful title instead of Cities of the Plain.)
For all initial appearances, Sodom and Gomorrah is more of the same. There is not the break that there was between past volumes: the transition from the flashbacks of Swann’s Way to Proust’s youth, the transition from Balbec to Paris. We are still in high society and, with the exception of a brief return to Balbec, the scenery is numbingly similar to that of The Guermantes Way. The apparent shift is mostly psychological, as Marcel has once more receded from the picture now that his infatuation with the Guermantes has expired. But there is a more subtle thematic shift–a nascent one–that hints at some resuscitation of life from the nihilistic attitudes of the prior volume.
Sodom and Gomorrah is divided into two parts: the first is thirty pages, and the second is five hundred. Without reading too much into this (Proust hadn’t finished revising the thing, after all), I’m still inclined to give equal weight to each. While the second is society-play of the sort seen before, the first is something else entirely. Marcel surreptitiously spies on M. de Charlus and the tailor Jupien, several times. They speak of homosexuality and engage each other as though belonging to a secret society, not as lovers but as conspirators, speakers of gossip who are allied with one another because of their kind. Proust makes a few observations about what he calls this breed of “man-women” which fall somewhere between stereotyping and species classification, but what’s more interesting is the process by which this discovery is made.
Marcel, in the novel, has never before eavesdropped on people to this extent; Proust’s omniscient knowledge of others has been treated as a given. Here, then, is something that has been consciously excluded from the novel so far. Homosexuality has surfaced at times (Odette, Albertine, Saint-Loup), but not as a principle of social order. In the women it was a dalliance; in Saint-Loup something to be rejected and ignored. But in Charlus, it’s a character factor that is so fundamental to his personality and his behavior that it, for Proust, separates him into another species:

This single new notion will bring about an entire regrouping, thrusting some back, others forward, of the fractional notions, henceforward a complete whole, which we possessed of the rest of the family…Now the abstraction had become materialized, the creature at last discerned had lost its power of remaining invisible, and the transformation of M. de Charlus into a new person was so complete that not only the contrasts of his face and of his voice, but in retrospect, the very ups and downs of his relations with myself, everything that hitherto had seemed to my mind incoherent, became intelligible, appeared self-evident, just as a sentence which presents no meaning so long as it remains broken up in letters arranged at random expresses, if those letters be rearranged in the proper order, a thought which one can never afterwards forget. (636)

The way Proust conceives this discovery, as an organizing principle, suggests that underneath the trivial chaos of social conceits, there are powerful and submerged forces, of which Marcel had not previously been aware. And permanence is a key aspect of his conception of homosexuality: for many pages, Proust has spoken of the fickleness of emotion and the alienation of people from their own past emotional states, but homosexuality, he implies, is something entirely different, a trait that underlies fundamental character, not a fleeting whim, and so is something that can be looked to for reference about a person’s deeper nature. Proust says nothing of this directly, but the contrast is difficult to ignore, and this treatment of homosexuality as more fundamental than love or any other emotion suggests that there may yet be some meaning to be found in the detritus of high society.
Of course, Proust then has this to say:

These descendents of the Sodomites…have established themselves throughout the entire world; they have had access to every profession and are so readily admitted into the most exclusive clubs that, whenever a Sodomite fails to secure election, the black balls are for the most part cast by other Sodomites, who make a point of condemning sodomy, having inherited the mendacity that enabled their ancestors to escape from the accursed city. it is possible that they may return there one day. Certainly they form in every land an oriental colony, cultured, musical, malicious, which has charming qualities and intolerable defects…I have thought it as well to utter here a provisional warning against the lamentable error of proposing (just as people have encouraged a Zionist movement) to create a Sodomist movement and to rebuild Sodom. For, no sooner had they arrived there than the Sodomites would leave the town so as not to have the appearance of belonging to it, would take wives, keep mistresses in other cities where they would find, incidentally, every diversion that appealed to them. They would repair to Sodom only on days of supreme necessity, when their own town was empty, at those seasons when hunger drives the wolf from the woods. In other words, everything would go on very much as it does to-day in London, Berlin, Rome, Petrograd or Paris. (655)

I’m tempted to just shrug this off as part of Proust’s general misanthropy, but it’s notable enough (especially next to his treatment of the Dreyfuss scandal, which I haven’t discussed at all yet) that it deserves mention, even without comment.

At the end of the first half of The Guermantes Way, Marcel’s grandmother dies. Over the course of fifty pages, she slowly degenerates, is treated by doctors, and finally passes on. It’s the most memorable sequence in the volume, and even without its surrounding material, it would be a remarkable deathbed sequence, sentimental but not mawkish, detached without being impersonal.
Marcel’s grandmother figured heavily in the second volume as a protective, benevolent figure, merging with the placid scenery of Balbec. She disappears for the first part of this book, only to reappear with a phone call to Marcel in which he, somewhat sadly, finally feels independent of her. Shortly thereafter, he is shocked to see her:

We never see the people who are dear to us save in the animated system, the perpetual motion of our incessant love for them, which, before allowing the images that their faces present to reach us, seizes them in its vortex and flings them back upon the idea that we have always had of them, makes them adhere to it, coincide with it…[here follows a verbose passage in which Proust seems to be putting off saying the inevitable]…And–like a sick man who, not having looked at his own reflexion for a long time, and regularly composing the features which he never sees in accordance with the ideal image of himself that he carries in his mind, recoils on catching sight in the glass, in the middle of an arid desert of a face, of the sloping pink protuberance of a nose as huge of one of the pyramids of Egypt–I saw, sitting on the sofa beneath the lamp, red-faced, heavy and vulgar, sick, vacant, letting her slightly crazed eyes wander over a book, a dejected old woman whom I did not know. (142)

And what follows from there is a private struggle between Marcel’s grandmother and her impending death, which is played out uncomfortably in public. Legrandin spies the family in a cab one day and gives a look of shock:

She had seemed to be foundering, slithering into the abyss, clinging desperately to the cushions which could scarcely hold back the headlong plunge of her body, her hair disheveled, her eyes wild, no longer capable of facing the assault of the images which their pupils no longer had the strength to bear. She had appeared, although I was beside her, to be plunged into that unknown world in the heart of which she had already received the blows of which she bore the marks when I had looked up at her in the Champs-Elysees, her had, her face, her coat deranged by the hand of the invisible angel with whom she had wrestled. (326)

Twenty-five pages later, it is continuing inexorably, but by this point Marcel’s grandmother is so central and so present in her final struggle that she seems to warp the environment around her to draw others in:

I found myself in the presence of a sort of miracle. Accompanied by an incessant low murmur, my grandmother seemed to be singing us a long, joyous song which filled the room, rapid and musical. I soon realized that it was scarcely less unconscious, that it was as purely mechanical, as the hoarse rattle that I had heard before leaving the room. Perhaps to a slight extent it reflected some improvement brought about by the morphine. Principally it was the result (the air not passing quite in the same way through the bronchial tubes) of a change in the register of her breathing. Released by the twofold action of the oxygen and the morphine, my grandmother’s breath no longer laboured, no longer whined, but, swift and light, glided like a skater towards the delicious fluid. Perhaps the breath, imperceptible as that of the wind in the hollow stem of a reed, was mingled in this song with some of those more human sighs which, released at the approach of death, suggest intimations of pain or happiness in those who have already ceased to feel, and came now to add a more melodious accent, but without changing its rhythm, to that long phrase which rose, soared still higher, then subsided, to spring up once more, from the alleviated chest, in pursuit of the oxygen. (352)

(Amazing passage, no?) Finally:

Who knows whether, without my grandmother’s even being conscious of them, countless happy and tender memories compressed by suffering were not escaping from her now, like those lighter gases which had long been compressed in the cylinders? It was as though everything that she had to tell us was pouring out, that it was us that she was addressing with this prolixity, this eagerness, this effusion. At the foot of the bed, convulsed by every gasp of this agony, not weeping but at moments drenched with tears, my mother stood with the unheeding desolation of a tree lashed by the rain and shaken by the wind…Suddenly my grandmother half rose, made a violent effort, like someone struggling to resist an attempt on his life…At that moment my grandmother opened her eyes…The hiss of the oxygen had ceased; the doctor moved away from the bedside. My grandmother was dead. (357)

I don’t have a lot to say about the sequence–it speaks for itself as a self-contained entity–but coming as it does in the middle of one of the driest parts of the book, its self-contained force obscures its connections to what surrounds it. The seemingly intentional striving for a dramatic, profound climax contracts with the apparent formlessness of what’s gone before. Underneath the polite nothings of the Guermantes and high society, the entire volume is, in fact, suffused with death and decay. Bergotte, aging and beginning to see the renown that most great writers never live to see, is a shell of man, and even Marcel’s affection for his work has faded. “Nice guy” Saint-Loup beats a man who solicits him for sex. Bloch is so alienated from his own faith and so attracted to high society that he ignores the implications of the Dreyfus case. And Swann, as is revealed at the end of the volume, is sick with cancer.