So let’s make a remarkably long story less long, and just summarize:
For starters, everybody’s partying at Castle Grigamore. It’s a nice time. Gareth and Lionesse are finally in the same room at the same time, after flirting long-distance a few chapters back, and it’s clear to everyone, not least themselves, that they are way into one another. They can hardly keep their hands off one another, they don’t pay attention to anyone else, it’s on the verge of unseemly.
Gringamore at one point pulls his sister aside and is like, “are you sure you want to throw yourself at this guy like this?”
To which Lionesse responds, “hell yes, he’s King Arthur’s nephew plus all the heroic stuff he did before.”
So Gringamore goes to Gareth, and is all, “my sister likes you.”
And Gareth is like, “duh. And I like her, too.”
And Gringamore is all, “so go ahead and hook up then and get married and have a million babies already.”
And Gareth is like, “no, no, because back a few chapters I promised to be a wandering knight errant for a year, and go to her afterwards. I’ll have to leave soon, for that.”
And Gringamore is all, “how about you and me and Lionesse all wander together for the year?”
Everyone thinks this is a great idea. I don’t know why Lionesse doesn’t just call off the year of errantry and skip to the wedded bliss, but whatever. The only fly in the ointment is Dame Linet. She resents how Gareth is pawing Lionesse and no one is pawing Linet; after all, it was only a few chapters back that it looked like Gareth and Linet were going to be one of those taming-of-the-shrew couples where the girl hates the boy and complains constantly and then suddenly for no reason has a total change of heart and becomes meek and submissive, like you see. When she overhears the two of them plotting to commit premarital sexhaving together, Linet is not down with it at all.
So that night, Lionesse sneaks into Gareth’s bed, and they’re getting hot and heavy… and then boom! A knight from out of nowhere! Gareth acts on instinct and smites the hell out of the poor dude before anyone knows what’s going on. He goes down! Blood! Blood! Lionesse screams! The knight, one of Gringamore’s men, burbles and moans! He’s dying! It is very much not sexy! Gringamore comes running, sees his sister and her beau in a state of undress, plus one of his guys lying on the ground bleeding. He freaks out! Sir Gringamore: useless in a crisis!
But it’s all good, because Linet saunters in, to the rescue! She arrives armed with a healing potion she put together for just this occasion, and applies it to the dying knight, and heals him right up, no permanent damage. So that night Linet’s scheme results in an innocent knight getting smited, and Lionesse and Gareth don’t have sex.
But the next night they totally get it on. And then this same poor knight appears! Linet keeps sending him in. How Linet gets him to do this isn’t clear. “Oh, Sir Arglebargle,” she maybe says, “I left my contact lens solution in Gareth’s bedchamber, would you mind going to fetch it for me?” That Gareth’s bedchamber is actually the dining hall repurposed may play a role here, but Malory doesn’t connect the dots for us.
Anyway, this time Gareth is ready for Sir Flim-Flam, and as soon as he sees him, he grabs his sword, smashes the knight’s skull to bits with a version of his brother Sir Gawaine’s patented Petulant Sneak Attack Skull-Smash style, toot sweet. Once again, though, it puts a pall over the lovemaking, and an alarm is sounded.
Over the course of the alarm, Linet quietly grabs all of the pieces of skull and brains and the knight’s corpse, and takes it back to her room and carefully reassembles him and anoints him with magic potion, and boom, he’s good as new. I don’t know why Linet failed to demonstrate this magical capacity earlier in the story! There’s certainly been plenty of opportunity.
Anyway. Gareth figures out that it’s Linet who keeps interfering in his and Lionesse’s attempts to hook up, and he’s annoyed at her for it. He’s also annoyed because at some point in their combats, this poor sap of a knight who keeps getting maimed managed to get in a good cut on Gareth, and now he’s wounded.