King Rience of Northern Wales has a brother, called Nero, just like the famous Roman emperor. Nero now battles Arthur and his knights, and although the former's army is numerically superior, Arthur's is just so much better that they win anyway.
However, apparently Nero was friends with King Lot of the Orkney Islands. If Lot had helped Nero, that would likely have been too much for Arthur. How fortunate for Arthur that Merlin continues his recent helpful streak; he goes to King Lot and starts prophesying so much that Lot becomes all distracted, and when finally he remembers that he was gonna help Nero beat Arthur and his knights, it's too late, Nero's army has been destroyed already.
King Lot, on hearing this, is ashamed (understandable). He decides to take his army and attack King Arthur even though it's too late to help Nero, thinking at least he can avenge his friend, and Arthur and his knights are gonna be really tired by now. Things don't work out as Lot planned though, since he's killed by Arthur's knight Pellinore, who chops his sword straight through Lot's helmet and head. (This doesn't seem feasible to me, but possibly Pellinore has a bit of super strength. More obvious superpowers soon to come in this story!)
Next, there's this big family meeting, since all those involved in these recent battles are family too. Morgan le Fay sees her chance to pull the old switcheroo on Arthur's magic invulnerability-granting scabbard. Morgan is married to King Uriens, but she's got a secret boyfriend on the side, and decides that he better be invulnerable. Arthur suspects nothing.
On his way home, King Arthur falls ill, and has his servants set up a pavillion where he can lay down and rest. A knight rides by, "making great dole". (In this day and age, men were not afraid of showing their emotions, that much is clear.) Arthur feels sorry for him, and asks him why he's so sad, but he won't tell, and just rides on. This pisses Arthur off! When Arthur wants to show compassion, he doesn't take no for an answer. So when he spots Sir Balin, Arthur orders him to chase down the doleful knight and bring him back, by force if necessary.
Balin does as he's told. He finds the sad knight in the company of his girlfriend, and tells him that he has to go back to Arthur and tell the king about his sorrows. The knight goes why? I will just feel worse doing that, and it seems completely pointless. (Reasonable.) Balin says look, you gotta come with me, you gotta tell Arthur about your sorrow, or else I'll have to take you back by force.
The knight doesn't want to make a fight out of it, and goes back with Balin.
Right when the two reach King Arthur's pavillion again, an invisible knight runs up to them and impales the sad knight on a spear. Seriously!
As the knight lies dying, he still manages to say a few last words to Sir Balin: The invisible murderer is called Garlon, and it is now up to Balin to finish the quest that the sorrowful knight was on before he was murdered. Balin should take his horse, which is a better horse than Balin's original one, ride back to the knight's girlfriend, and she'll inform him about the quest. Also, his name is Sir Herlew le Berbeus.
King Arthur, who sadly never got the chance to hear Herlew tell him all about his sorrows, now bury him instead, whereas Balin dutifully takes up this new quest.