Primary Sources: Le Morte D’Arthur, Book IX Chapter 17
Tristram and Lamorak meet back up at the forester and collect Kehdyius, and then the three of them ride back to the ship where they’d gotten wrecked back in the first place, and meet the folks they left there, Gouvernail and Bragwaine. Then they all sailed to Cornwall together.
“But wait!” is what you’d be protesting if you were paying sufficient attention, which, it’s okay, that’s why I’m here. “Wasn’t the ship wrecked, and didn’t the knights go out into the woods in order to scout out safe passage for the others, because they were going to need to find alternate transport? Did the ship just magically fix itself?”
To which we can only answer, apparently! Apparently it did magically fix itself. Do you have any other questions?
“While we’re at it,” you say, “wasn’t Isoud the White on the ship with them? Didn’t she get left behind along with Gouvernail and Bragwaine?”
To which we can only answer, congratulations! You’ve failed to forget about Isoud the White, which puts you one up on Malory! As near as I can tell, Malory just plain forgets she exists, because she’s never mentioned or brought up again after this chapter.
Anyway, the whole gang sails to Cornwall, and to King Mark’s court at Tintagil. Long story short, Isoud and Tristram immediately pick up where they left off, with regards to their illicit affair. Carrying out the affair takes a little bit of effort, as Mark is still around. As far as he knows, Sir Kehdyius came to Tintagil all by his lonesome. Luckily, Sir Dinas, Mark’s seneschal, has no respect for his liege. Dinas helps them out by covering for Tristram, and arranging for Isoud and Tristram to be alone together. During the logistics of smuggling them around, Sir Kehdyius meets the lovely Isoud for the first time, and falls for her immediately. Don’t worry, Malroy tells us: eventually he dies for her.
Kehdyius writes a series of letters to the lovely Isoud, declaring his love, explaining that he knows that she’s married to Mark and is the illicit lover of Tristram when she’s not being “abducted” by Sir Palomides, but if there’s space on her dance card for a fourth-in-line knight, he’d be much obliged.
Isoud writes back to him with a series of noncommittal but sympathetic-sounding notes. “You poor dear,” that sort of thing. Somehow Tristram gets ahold of one of these notes, leaps to all the wrong conclusions, and goes off on a tear at the lovely Isoud, confronting her and Kehdyius both.
“Alas Madam the good love that I have loved you, and many lands and riches have I forsaken for your love, and now ye are a traitress to me, the which doth me great pain.”
“Forsaken… do you mean we’re not going back home to Brittany?” asks Kehdyius.
Tristram has some choice words for Kehdyius, all about the various good things Tristram did for Kehdyius’s family (slaying Grip, saving Kehdyius from the shipwreck, accepting rulership of Brittany, marrying ol’ what’s-her-name and only going to second base). At the climax of his rant, he pulls out his sword and starts waving it around, which naturally leads to Isoud fainting. While Tristram is distracted by the lovely Isoud’s fainting spell, Kehdyius dives out a nearby window!
He lands on top of King Mark, who’s spent this whole chapter hanging out in the garden playing chess with himself. Mark is extremely curious as to why Kehdyius fell from a second-story window onto Mark, but accepts Kehdyius’s explanation about how he was leaning out the window admiring the view, and then fell asleep. (Malory!)
I’m guessing Isoud the White was cleaning Tristam’s sword when it accidentally went off and decapitated her.