Guenever spends the night in a chamber just outside of Sir Meliagrance's. She's in the inner chamber, while her wounded knights are in the outer.
Lucky for Lancelot, there's a window in her chamber, without glass.
The two talk for a time through this window but no one really knows what they talked about.
Then, Lancelot tells her he wants to come in and be with her, and she agrees. Using his super-human strength, he bends the bars and comes right in.
The next morning, Sir Meliagrance is eager to have Guenever off his hands. He loves her, but can't have her, so just wants her gone now.
He can't help himself from going into her room to wake her up, though. And when he enters, he sees blood on the sheets (from Lancelot's cut hand).
Suddenly, Meliagrance is accusing Guenever of being a traitor to the king. He thinks she's been sporting around with the wounded knights during the night.
Lancelot shows up, right on cue, and tells Meliagrance that he'll defend the Queen—that he'll be her champion to prove that none of the knights who stand accused are guilty. Nice sidestepping of the issue, Lance.
So, this will be the second time out of three that Lancelot will save Guenever from being burned at the stake (the book tells us, in a moment of foreshadowing).
Meliagrance gets Lancelot to agree that they'll be good friends until the date of their combat.
He then takes Lancelot on a tour of his castle. And promptly locks him up.
No one thinks anything of this because of Lancelot's long tradition of making sudden disappearances and appearances.