Catelyn finds out Robb is planning to march somewhere, but not to Harrenhal. He sends her to go and talk to Renly who is on his way to King's Landing.
Renly has a vast host, doesn't appear to be in any hurry, and lets his knights play at tourneys. He likes to dress up and have parties and be charming. This means he is probably going to suck at battles. It says a lot that his top knight is Ser Loras Tyrell, the handsome flowery one that Sansa fancied. Loras gets beaten by a girl, Brienne. Tough tomboys seem to be a bit of a GRRM theme.
Catelyn tries to get Renly to join forces with Robb but Renly isn't having it unless Robb bends his knee. Renly seems a bit overconfident in his numbers. My guess is they've been charmed and well fed by him, but as soon as he does badly in a battle they'll all desert him.
Stannis has attacked Renly's castle, Storm's End.
