Catelyn is cuh-razy with grief over Bran; she refuses to do anything but stay by his bed, even though Robb points out that she has other children and other duties. (The youngest child, Rickon, is only three and he's crying all the time.)
Robb has to leave his mother and Bran alone when a fire starts in another building. This is when an assassin comes to kill Bran. Luckily, Bran still has a wolf and wolves love assassin-meat.
After the assassin is killed, Catelyn comes to her senses: she knows there are things to do, and first on her list is finding out who tried to kill her Bran – twice. (Everything's always about Bran with this lady.) She and Ser Rodrik Cassel (master of arms) decide to sail for King's Landing to find out who ordered the assassination. Their one clue: the assassin used a real fancy dagger.