Sansa is pleased that Joffrey is dead, but realises that she might get the blame. Anyway she is being rescued by Dontos. He had told her to wear a "magic" hairnet encrusted with black amethysts. Sansa discovers one is missing and thinks this may be the poison that got Joffrey. This would make Dontos the murderer. It seems an extreme step just to help Sansa escape from the wedding.
They climb down the cliff and into a waiting boat. They row out to sea and Sansa gets onto a ship. To find none other than Petyr Baelish waiting for her. He's a blast from the past! Where has he been all this time? The big shock of this chapter is that Dontos was selling Sansa, not rescuing her. He gets paid in crossbow bolts. As Sansa says, *everything* is a lie.
By way of explanation Petyr says a lot of cryptic stuff about Catelyn giving him her daughter. It seems like Petyr is playing the game of thrones: he must have quite the long con going on somewhere. I can't see what it is yet. I wonder if Tywin is really behind all this. Perhaps he is going to marry Sansa to Bolton: that would explain his comments back in chapter 53, when he discussed finding a bride for Bolton, about Littlefinger (Petyr) knowing where Arya is. Sansa would suit just as well. Then again, she is still married to Tyrion, so there must be more going on.