First of all, a bit of genealogy. Sirs Agravain, Gawain, Gareth and Gaheris are the sons of King Lot of Orkneys and Morgause, Arthur’s half-sister. Sir Mordred is an illegitimate son of Arthur and Morgause, who once visited him in his bedchamber, trying to plead for her husband’s life, as he started an unsuccessful rebellion against Arthur. They were apparently ignorant of their familial relationship (don’t ask me how). So Mordred, whom I have known so far only as Arthur’s nephew, is also his son, although I’m not sure whether they’re both aware of it (can’t resist quoting at this point Chinatown’s “She’s my sister! She’s my daughter!” scene). Anyway, all these guys are now the Knights of the Round Table, and while all the G-men are nice guys, Mordred and Agravain are nasty, because they hate the Arthurian’s cycle Unofficial Couple, i.e. Lancelot and Guinevere and they’re plotting to bring them down.
On a side note, I’m sure Mordred and Agravain are supposed to be aptronyms. Mordred, Mordred – have you ever noticed how the name sounds like “murder”? I know it mutated in various versions and languages but I’m certain Malory at least semi-consciously picked the version that sounded most ominous. As for Agravain, it is pretty obvious, as in Middle English “aggrieve” was “agreven”. And of course the alliteration in the names of the three good guys (see what I did there?) is also on purpose.
So one day in the locker room in the castle in Carlisle, or wherever the knights were supposed to gather (actually, in the king’s chamber), Agravain says aloud “Lancelot sleeps with Queen Guinevere and somebody needs to tell the king about it”. Gawain, backed by his two G-brothers, pleads with him and Mordred not to do it: Lancelot is the greatest of all knights, all of them, Mordred and Agravain included, owe their lives to him and so on. But Agravain and Mordred insist on it and the G-knights leave slamming the door, as they don’t want to be a part of it, wailing on their way and predicting darkly what is going to become of the kingdom. At this point Arthur notices the commotion [was he sitting by himself at the other end of the room? weird] and asks what’s the deal. Mordred and Agravain tell him what Arthur has been secretly suspecting himself for a long time but didn’t want to admit to himself because Lancelot was his best knight and best friend. He says he won’t believe it unless Lancelot is caught red-handed, because if they just accuse Lancelot, he’s going to challenge them to a duel, which they will surely lose.
Agravain and Mordred then suggest a stake-out. Arthur is to ride out hunting and in the evening send the message that the hunting got prolonged and he won’t be back in the castle for the night. Meanwhile, they with twelve other knights are going to wait for Lancelot outside queen’s bedroom. The twelve knights are all carefully listed by name and as the writer specifies, they were all either Agravain’s relatives, or supporters of his brother, or from Scotland [some Anglo-Scottish animosity there?] Lancelot of course is getting ready to spend the night with Guinevere but Sir Bors, Lancelot’s nephew and confidant warns him – he knows that Agravain has some bad intentions and he is having particularly strong misgivings about tonight.