Primary Sources: Le Morte D’Arthur, Book XI, Chapter 8
That night, as she promised, Brisen uses what looks like the same magic Merlin used way way back in the beginning of Book I, and disguises herself as one of Guenever’s handmaidens. She sneaks into the knights’ barracks and finds Launcelot’s cot, and wakes him up.
“Psst! Sir Launcelot!”
“My lady, Queen Guenever, lieth and awaiteth you!”
“Awesome,” says Launcelot. He’s been waiting for this, and has his long nightshirt and sword all ready.
Launcelot does not go anywhere without his sword.
Brisen leads Launcelot by the hand through the darkened castle, arriving across the hall from Guenever’s room. If Launcelot thinks it odd that Guenever has moved across the hall, he doesn’t comment on it. Then Brisen leads him into Elaine’s bedroom, and right to Elaine’s bed, and once he’s climbed into bed, she leaves.
Elaine and Launcelot are alone together! And Elaine is thrilled! And Launcelot (who is not an idiot and realizes in a few seconds that Elaine is not Guenever) is likewise pretty thrilled, because Elaine is hot and super into him and naked and right there.
Malory’s actually weirdly ambiguous about this: he weened that he had had another in his arms, is what he says. “To ween” means to incorrectly suppose something, but who is “another?” Maybe that means Launcelot understands that he has another (i.e. someone who isn’t Guenever) in bed with him, and therefore is not presented as being mind-controlled or mistaken or anything. Or maybe it means that as far as Launcelot understands it, he has another person besides Elaine in bed with him (i.e. Guenever), and he’s just unable to distinguish between Guenever (who must be getting up there in years) and teenage Elaine. There’s no indication that Brisen is disguising Elaine even slightly, or that Launcelot’s senses are impaired. The first time I read it, it seemed clear that Launcelot knew he was sleeping with Elaine, but now I’m unsure.
Meanwhile! Guenever sends her actual handmaiden, the one that Brisen impersonated, to fetch Launcelot. The maiden sneaks into the knights’ barracks, as she has many times before, but inside: no Launcelot. She rushes back to her lady’s chamber, and tells Guenever.
“Alas, where is that false knight become?” Guenever asks, rhetorically. She freaks out, and considers waking the whole castle to search for Launcelot, but that seems like a nonstarter, what with their affair being nominally a secret. So instead she just tosses and turns in her bed, and doesn’t sleep at all, for hours.
Meanwhile! After their lengthy and athletic lovemaking, Launcelot and Elaine fall asleep in one another’s arms. And here’s something Malory bets you didn’t know about Launcelot: he talks in his sleep. Not just that, he talks in his sleep loudly and at length, mostly about his affair with Guenever.
This doesn’t wake Elaine, but Guenever, already awake across the hall, hears him. In a flash she realizes that Launcelot is in bed with Elaine, the very thing she had tried to prevent! She is nigh wood which is an idiom that we don’t use any more and also out of her mind which is an idiom we do use still! I believe “nigh wood” means that she responds so strongly she puts one in the mind of a wooden puppet making hugely exaggerated emotional displays for a Punch & Judy show, but that’s just a guess.
Guenever lets out a loud noise, which Malory variously calls a cough and a hemming and which I’m settling on thinking of as a groan of anguish.
Guenever’s noise wakes Launcelot up, and in a flash he either realizes that he just slept with Elaine and not Guenever, or (and I think this makes a lot more sense) he’s suddenly overcome with crippling guilt over having yielded to the temptation of an eager naked teen girl. In either case, he leaps up out of bed and rushes into the hallway, where Guenever stands waiting for him.
“Get out of here, false traitor knight,” Guenever says, before Launcelot can even try to explain himself. “Never abide in my court, and avoid my chamber. You are not so hardy, you awful man, that you’ll survive appearing in my line of sight ever again. That’s right, I just threatened you, Sir Launcelot, with violence. That’s how upset I am. I am upset.”
Launcelot doesn’t try to defend himself; he just collapses sobbing.
Guenever leaves, and after a few minutes Launcelot picks himself up and jumps out the nearest window.
Fortunately he falls into a garden full of nice soft grass!
Unfortunately he misses the grass and lands in a big thorny rosebush!
Long story short, Launcelot, bleeding from the heart and also from a lot of minor cuts and thorn-scratches, stumbles away from Camelot in his nightshirt, clutching his sword. He moves out into the woods and becomes a hermit, where his hair grows long and he gets all crazy.
Well, there goes the credit I gave him just a few chapters back. Way to go, dude. That faithfulness (to an adulterous relationship, but faithfulness nonetheless) sure didn’t last long.