Robert dies and Cersei declares Joffrey king. Eddard goes to confront her with Robert's will, but Cersei just rips it up. (Why didn't we see that coming?)
So Eddard declares that Stannis is the rightful heir and he asks Janos Slynt, captain of the City Watch, to arrest Cersei. But the City Watch's aim is apparently terrible and they arrest Eddard instead (and kill all his men – again).
Apparently, Petyr betrayed Eddard. He tells him: "I did warn you not to trust me, you know" (58). That's "Betrayal" at its worst right there.