Becca actually beat Launcelot to the abbey, since Launcelot was delayed a chapter with the comic/homophobic misadventure.  She greets him with a hug and makes sure he gets a shower and a change of clothes before sending for her father, King Bagdemagus.  When Bagdemagus comes, he brings a pack of his own kinghts with him, just in case, but when he sees Launcelot he smiles and waves and they embrace.

“Launcelot, my good man, what the heck has happened?”

“It’s a long story,” says Launcelot, and recounts the events of the first five chapters of the book to Bagdemagus (excepting the parts of Chapters I and II where he was asleep).  “So, to sum up, I don’t know where Sir Lionel is, and I owe Becca (or Becca’s father) a favor.”

“And I’ll call in that marker, yeah,” says Bagdemagus.  “There’s a joust next Tuesday…”

“Becca told me about it.  So you and the King of Northgalis have a regular joust thing going, and he’s called in some of King Arthur’s knights on his side?  Who’s he got?”

“Some of the younger knights.  No one important, like you would have heard of: there’s Sir Mador de le Porte, Sir Gahalantine, and Sir Mordred.”

“Mordred?”

“Enh, that’s what’s on the card.”

“Well, here’s what we do.  I’ll get a face-concealing helmet, and you get three good knights of yours with matching helmets, and we’ll all four wear shields that are plain white, and fight as a unit, and it’ll be hard for the opposition to tell that one of your knights is much much tougher than the others, plus they won’t know that it’s me.”

And this is what happens, long story short. Laucelot and King Bagdemagus’s men joust in a big tournament against King Northgalis’s men and Northgalis’s three ringers from Camelot, and Launcelot is disguised as a generic guy.

The three ringers — Mador, Mordred, and Gahalantine — hang back and watch the initial bouts.

“One of those knights of Bagdemagus’s is pretty nasty,” observes Mador.  “Hold my beer, I’ma go joust him.”

This goes poorly for Mador.

“Now it’s Mordred’s turn to shine!” cries Sir Mordred.  He charges Launcelot and they joust and Launcelot whups him so hard Mordred goes flying through the air and lands on his head.  His helmet breaks in half and he breaks his neck and he’s in a coma for weeks, Malory tells us.  Mordred eventually more or less recovers, but he’s never again quite right in the head. (Way to go, Launcelot! Camelot might have continued indefinitely were it not for you damaging Mordred’s brain!)

But that’s all afterwards, at the time he just goes down and Gahalantine finishes his drink and jousts Launcelot and loses, although not as quickly as Mador and not as messily as Mordred.

Once the three ringers from Camelot have been dispatched, Launcelot rides through, just laying into Northgalis’s knights, until finally Northgalis signals a surrender. Bagdemagus and his men cheer, because they’ve been doing this every Tuesday for I don’t know how long, and they always lost up until this point.  It’s also at this point that someone gets Mordred some medical attention.

Everybody goes off to either Northgalis’s castle or Bagdemagus’s castle, and in Bagdemagus’s castle Launcelot is the hero of the hour.  The next morning Launcelot says he’s leaving to go search for Sir Lionel, but he takes Becca aside and tells her, if you ever need a knight, you get the word to me, and I’ll come running.