Sir Persant, the Indigo Knight, gets tired of waiting around on Gareth and Linet to ride up, and rides out to meet them, and long story short there’s a joust and Gareth beats the Indigo Knight.
Afterwards the Indigo Knight reveals that the Black Knight, the Green Knight, and the Red Knight were all his brothers (their names are Sir Percard, Sir Pertolepe, and Sir Perimones), and swears fealty to Gareth as his two surviving brothers had before him.
They have a big feast, and afterwards Gareth retires to bed and the Indigo Knight decides to send a girl to his bedchamber, specifically his eighteen-year-old daughter and if you think this is kind of out of left field you aren’t alone. She’s a dutiful daughter and does as he commands. When she gets to Gareth’s chambers he’s already asleep, so she strips naked and climbs into his bed and wakes him up.
Once Sir Gareth realizes he isn’t dreaming he backs up and asks the girl what the heck is going on, and she explains that she’s the Indigo Knight’s daughter, and her father sent her to Gareth’s bed.
“Are you a virgin, or a slut?” Gareth asks her. Seriously. Malory phrases it a little differently, but only a little: Be ye a maid or a wife?
The girls’ response leaves no room for misinterpretation: sir, I am a clean maiden.
“Okay, well, I’m not about to shame your father by deflowering, or as Malory says, I won’t defoil you.” Which sounds like a step in making Jiffy-Pop or a TV dinner, defoiling. “Get out of here, go on,” he says, but he says it kindly and kisses her on the forehead.
The girl runs back to her father (I would assume she pauses to put her clothes back on but you never know) and fills him in on how it all went down.
“Well, one thing’s for sure — he’s a gentleman, and must therefore be of noble birth,” says the Indigo Knight.
(Again: Sir Thomas Malory, clearly not a feminist even by the standards of the fifteenth century)
The next morning, over breakfast, the Indigo Knight finally gets around to asking Linet what her deal is, and she explains that she’s taking Gareth out to her sister, specifically to her castle and the siege it’s been under for two years. Salient points:
1) Linet’s sister lives in a castle called Castle Dangerous, which, I gotta say, is right up there with rescuing Princess Damsel from the Red Dragon of Reddragonville.
2) Over two years have passed since the Red Knight of the Red Lands — who is a totally different Red Knight of the Red Lands than the one that Gareth already met on this same adventure — started his siege of the castle. The fact that there are two Red Knights in this story is deftly sidestepped.
3) Linet’s name is Linet, someone finally gets around to mentioning, and her sister’s name is Lionesse.
4) LInet requests the Indigo Knight knight Gareth, so Sir Prettyboy will be a legit knight, but Gareth explains that Launcelot already knighted him, which leads him and the Indigo Knight to list off a bunch of great knights and talk about how great knightliness is.
5) Gareth reveals for once and for all that he’s Gareth, the youngest son of King Lot and Queen Morgawse, brother of Gawaine, Agravaine, and Gaheris. He self-identifies as the youngest brother, which, I dunno whether that means he’s younger than Mordred or whether he knows that Mordred isn’t Lot’s son.