While Gareth gets a well-earned night’s sleep at Peter’s lodge, Lionesse is scheming!  She’s back at her own castle, and it very much vexes her that she doesn’t know exactly who this “Sir Prettyboy” is.  From Peter’s description back in Chapter XIV, she knows the following:

A) He is a legit knight.

B) He’s defeated a bunch of different colors of knight, also jousted Launcelot to something like a draw.

C) He’s traveling incognito from Camelot.

She doesn’t know his name, which really bugs her.  She also has forgotten that Peter explicitly said he was the son of old King Lot of Orkney, because she complains to her brother, Sir Gringamore, about how she really knows nothing about where Prettyboy is from and who his family is and so on.

Sir Gringamore, brother of Lionesse and Linet, appears abruptly in the narrative at this point.  I suppose he had been hiding from the Red Knight, or something.

“Here’s what I want you to do, Gringy,” says Lionesse.  “Go follow Sir Prettyboy, and surely he’ll lead you directly to Peter the dwarf.  Linet told me that Peter knows who Prettyboy’s father is, in another of Malory’s beloved continuity errors, I don’t know that, so, go, kidnap Peter for me and bring him back to your castle.  Linet and I will meet you there, and we can question him.”

“Check,” says Sir Gringamore.  He dresses up in his best sinister black arms and armor, and rides after Gareth, and finds him sleeping at Peter’s house, and Peter there too.

“Yoink!” he cries, and scoops Peter up and carries him off.  He tucks Peter under his arm like a football, apparently.

“What the —!  Help!  Help!  I’m being abducted!” shouts Peter, which wakes Gareth up.  Gareth sees Gringamore riding off with Peter, but by the time Gareth has his armor back on and stuff, Gringamore’s out of sight.

Gareth rides off after him, gets lost, gets directions from a helpful peasant in a wholly pointless interlude, and arrives at Gringamore’s castle.

Gringamore and his sisters are already there, of course, along with their prisoner Peter.

“I demand to know Sir Prettyboy’s real name!” cries Lionesse.  “Tell us!  Tell us, or else you will be pent up here for all time!  My brother’s got a great dungeon, you’ll love it there, oh no wait you won’t because it’s terrible!”

“Wait, what is this about?” asks Peter.  He’s understandably peeved.

“Prettyboy — who is he?  Who is he really?  Where was he born?  Who are his parents?”

“I… why didn’t you just ask me?  I have no reason not to tell you.  I mean, I already told you this once before.  He’s Sir Gareth, youngest brother of Sir Gawaine, son of King Lot and Queen Margawse, nephew of King Arthur.  Did I not make that clear once already?  Anyway.  Now that that’s settled, how about you let me go and we can just forget this happened?  Because otherwise Gareth is liable to show up and tear this castle down, I’m warning you.  Jerks.”

“I was going to let you go, but then you called me a jerk,” says Gringamore, all sanctimonious.  “Let’s all go to dinner without Peter and let him think about what he’s done,” he says to his sisters.

And the three of them leave Peter locked up, and go to supper in Gringamore’s castle’s dining hall, where it’s all happiness and light, since Dame Lionesse is finally out of her castle and that stupid siege lifted.  Linet points out that Prettyboy probably is a king’s son, given how nicely he treated her, which makes sense according to 15^th century logic I guess.

Gareth shows up, finally, and shouts outside the castle for Gringamore to appear and relinquish Peter.  Gringamore goes over to a window and shouts down about Peter is Gringamore’s prisoner and will be kept locked up until he learns his lesson.  Gareth offers to joust over it, which Gringamore thinks is an awesome idea, but Lionesse would rather just put this whole sordid interlude behind them and move on, so Gringamore reluctantly gives up Peter.

Lionesse apologizes to Gareth for abducting Peter, and Peter and Gareth reunite, and Gareth’s like, man, what a stupid misadventure this was, and Gringamore invites them in for after-dinner drinks.