By chapter 4 of part 2 of Sodom and Gomorrah, Proust’s narrator has all but given up on his relationship with Albertine. He’s decided that it would be madness to marry her, a sentiment that gratifies his mother, and he’s set his sights on Andreé, dreaming up a scheme to win her love. All that’s left is to cut off things with Albertine. She and the narrator are on a train coming back from La Raspelière discussing their dinner with the Verdurins (as the narrator has decided to put off breaking up and the serious conversation involved) when he mentions, in regard to the composer, the name Vinteuil.
The passage starts here:

“We may have revolved every possible idea in our minds, and yet the truth has never occurred to us, and it is from without, when we are least expecting it, that it gives us its cruel stab and wounds us for ever.” (p.701)

What is the “truth”? The narrator circulates thoughts and anxieties about Albertine’s character and sexuality throughout this volume. In Swann’s Way, Vinteuil’s sonata serves as a leitmotif for the relationship between Swann and Odette–a relationship tinged with promiscuity, deception, and lesbianism. In the same volume, in the narrator’s childhood, while at Montjouvain, the narrator observed Mlle Vinteuil and her mistress in the act of hooking up (and the act of sadism). In fact, as it turns out, Albertine is on extremely close terms with both Mlle Vinteuil and her lover (“oh! not at all the type of woman you might suppose!”, p.701) and is about to set out on a cruise with the latter (“it sounds a bit weird, but you know how I love the sea”, p.701). It seems like Albertine is going out of her way to vouch for the virtue of these women, even though she doesn’t know about the scene of sadism witnessed by the narrator. In fact, the mention of Mlle Vinteuil and her mistress triggers an involuntary memory of the episode for the narrator:

“At the sound of these words, … an image stirred in my heart, an image which I had kept in reserve for so many years that even if I had been able to guess, when I stored it up long ago, that it had a noxious power, I should have supposed that in the course of time it had entirely lost it; preserved alive in the depths of my being–like Orestes whose death the gods had prevented that, on the appointed day, he might return to his native land to avenge the murder of Agamemnon” (p.702)

In this section, we are shown that involuntary memory isn’t always pleasant. The memory of the scene is compared to the mythological figure of Orestes, who, along with his sister Electra, avenged the murder of his father Agamemnon at the hands of his mother Clytemnestra and her lover Aegisthus. The narrator wonders what he’s done to deserve the psychological slaying delivered unto him by the knowledge of Albertine’s connection to a “practicing and professional Sapphist” (p.703)–is it retribution for allowing his grandmother to die? Or maybe for viewing the scene way back at Montjouvain in the first place, “to make dazzlingly clear to my eyes the fatal consequences which evil actions eternally engender, not only for those who have committed them but for those who have done no more, or thought they were doing no more, than look on at a curious and entertaining spectacle” (p.702). The narrator still gets a little kick of pride and joy from achieving involuntary recollection and finding some lost time, because that’s the point of the book.
The narrator compares the gravity of this realization in relation to his previous doubts to the gap between early unimpressive telephone prototypes and the ultimate interconnection of cities and countries by miles of sweeping wire. The truth (or at any rate, the possibility of truth) behind Albertine’s relationships with women dwarfs the uneasy feelings, thoughts, and doubts that have plagued the narrator up to this point when he sees Albertine’s interactions with her friends, although it continues along the same lines–”this deluge of reality that engulfs us, however enormous it may be compared with our timid and microscopic suppositions, has always been foreshadowed by them” (p.703). The idea presented here is that the human mind can catch glimpses of hidden truths through guesses or suspicions that precede reality–which often dwarfs first impressions, just as many visitors to the 1889 Exhibition who could imagine sound transmitted across the length of a house couldn’t have imagined the to-be ubiquity of the telephone.
Proust writes that “[it] is often simple from lack of creative imagination that we do not go far enough in suffering” (p.703). Remember the story of Swann and Odette’s courtship, when Swann received the anonymous letter detailing Odette’s sexual past and was more upset that someone he knew would send him an anonymous letter, because he couldn’t find verisimilitude in the accusations–he couldn’t imagine any of the listed trysts. To finish the passage:

“And the most terrible reality brings us, at the same time as suffering, the joy of a great discovery, because it merely gives a new and clear form to what we have long been ruminating without suspecting it” (p.703).

Proust is talking about what I would call closure. It’s harrowing to ruminate for ages on the possibility of some horrible truth without getting any closer to it, so to finally learn even an awful fact is relieving, and even joyful, since it’s a discovery of something one might’ve known all along. For the narrator, the discovery of Albertine’s potential lesbianism is one he’s been postponing for most of the volume, and apparently provides him with so much joy and closure that he ends up deciding that he absolutely must marry her. Whatever, Marcel, it’s your life, write it whichever way you like.