Cersei meets Ned in the gardens of King's Landing and suggests that maybe it's time for him to go back to Winterfell, where he's less likely to get stabbed in the leg like that. I think she's right, but Ned has something else on his mind, because he's finally figured out what Jon Arryn was killed for. He knows that Cersei and Jaime are lovers. Cersei does not deny it, which I find kind of surprising. In fact, she says that the Targaryens wed brothers and sisters for three hundred years, although she doesn't add that that's a good way to end up with a dope like Viserys as the last scion of your once-mighty dynasty. It sounds to me like Khal Drogo is exactly the sort of person that bloodline needed. Cersei says that she shared a womb with Jaime, which gave them a special bond. Lots of twins share that bond, and most of them, as far as I know, don't end up having sex. Ned has connected all the dots: he's figured out that Bran must have seen them together, and also that all the Lannister children are Jaime's, not Robert's. That brings up an interesting angle to Tywin's worries about the family name. Even if Joffrey becomes king, he'll be Joffrey Baratheon. His blood is all Lannister, but for there to be a family called Lannister, Jaime and Tyrion need to be out there fathering some legitimate children.
Anyway, now that the whole story's out there, Cersei's perfectly happy to give more details, like how she "finishes Robert off in other ways" to prevent the chance of him accidentally impregnating her. She told Catelyn that she had a black-haired baby who died of some disease; I wonder if she was lying. Or if she killed the baby because it wasn't Jaime's. I guess we can decide for ourselves how evil Cersei's supposed to be. Although she does say she used to worship Robert. She was positively giddy in love. And then, on their wedding night, he whispered "Lyanna" in her ear, which I should remind you is the name of Lord Stark's dead sister. So that made Cersei hate Robert, and probably hate the Starks, too. Ned tells her to get out of town before Robert gets back from hunting. And to take her kids, too, because Ned doesn't want to be responsible for them getting killed when he tells Robert everything. Cersei says that Ned should have just become king himself. When he made Jaime give up the throne, it was right there for the taking. But he stepped back and let Robert be king instead, and we can all see how well that worked out. She adds, "When you play the Game of Thrones, you win. Or you die. There is no middle ground." She leaves the garden, and I have to wonder if Ned's going to take her seriously. Or does he think that all he has to do is tattle on her to Robert, and then everything will be all right somehow?