Karna begins the 17th day of the war by fighting each Pandava except Arjun. He is actually defeated by both Yuddistira and Bhima, but then he defeats Nakul and Sahadev. Both of the elder brothers spare his life, out of consideration for Arjun's oaths. Yuddistira then goes for a second round with Karna, but this time it doesn't work out so well.
Karna defeats Yuddistira and nearly captures him, but he reconsiders and lets the Pandava go. Yuddistira then flees to his camp to have his wounds tended to, and there he slips into a strange funk. What follows seems completely bogus to me. Arjun retreats to the camp to check on his brother's health, and Yuddistira perversely assumes that he must have already killed Karna.
Yuddistira admits that he's been terrified of Karna and hasn't slept for 13 years out of fear for him. When he finds out that Karna isn't killed, he gets really mad at Arjun and suggests that Arjun hand his bow to Krishna and that they change places. Arjun takes great exception to this, and pulls an oath out of his ass that he says he swore when he was younger. This oath was to kill anyone who suggested that he give up his Gandiva bow. So now he must kill Yuddistira.
Pretty lame, eh? Krishna is there, and he does his usual thing, by coming up with a "workaround", in which Arjun need only insult his brother, then brag about himself.