Arthur has to go help King Benwick (Lance's father) fight off another evil king, to pay him back for helping Arthur defeat Lot. He asks Lancelot to stay in England to be in charge. There's going to be all that difficulty soon between Mark of Cornwall and Tristram (and if that's not some heavy-handed foreshadowing, we don't know what is).
Plus, Arthur needs someone to look after Guen.
Say. No. More. Lance has got this.
Just kidding—he asks Arthur to please consider someone else for the job. And, of course, Arthur trusts no one as much as Lancelot.
We get a teeny flash-forward here: Lance and Guen will continue their affair for twenty-four years, but this one year while Arthur is away is really the only one in which they will find something close to happiness.
We see scenes of them talking, kissing, playing, fighting, and telling each other their deepest secrets.
Lancelot confesses to Guen that when he was younger, he wanted to do miracles, because it was not enough for him to be the best knight on earth—he wanted to conquer heaven, also.
He then tells her about performing a miracle—rescuing a maiden from boiling water. Guenever doesn't like this one bit. She, she tells him coldly, has never performed a miracle.
Lancelot tries to smooth things over by telling her that he's just trying to share the daydreams of his youth with her.