Catelyn is reunited with Robb (and his army) and she really wants to send him home to be safe, but she knows she can't because he's kind of in charge now.
She explains to him that Sansa's letter was dictated by Cersei and that they have to beat the Lannisters if they want to save Eddard, Sansa, and Arya. (They think that the Lannisters might have Arya, though Sansa's letter strangely did not mention her sister.)
The Lannisters have badly beaten the Riverlands (the Tully part of the Seven Kingdoms) and destroyed Beric Dondarrion's force that Eddard sent out to beat Gregor's raids. But Robb has a plan that involves a decoy (led by Roose Bolton) and a surprise attack (led by, oh, himself –what's the point of commanding an army if you can't have a little excitement along the way?).
Robb wants Catelyn to go home (apparently everyone wants to send everyone else home), but she says she needs to see her dying father.