Sansa is a bit fed up at the Eyrie, since there isn't a lot going on and she doesn't have any friends. She builds a snow-Winterfell, but Petyr joins in and then kisses her. I'm pretty sure he knows Lysa will see and get jealous, and sure enough that's what happens.
Lysa is really a crazy cat lady. She summons Sansa and blames her for seducing Petyr. Then drags her to the Moon Door out of which is a sheer 600 foot drop. It's the door Tyrion would have been dropped out of had he lost his trial after Catelyn brought him to the Eyrie. Petyr turns up and saves Sansa.
We learn that Lysa wanted to marry Petyr before, and almost gave him a son, but Lord Hoster murdered the son (tansy being one of the ingredients of the poison, all of which explains Hoster's death-bed murmerings). Lysa spills the beans about Petyr telling her to kill Jon Arryn, which is how Ned Stark became King's Hand right at the beginning of all this. We also learn that Petyr told her to tell Catelyn that she suspected Lannisters of killing Jon Arryn. I'm not sure what this achieved apart from Ned doing some investigating: perhaps that helped him to get in trouble and get killed. I'm still not entirely sure what Petyr is trying to do but it is very complicated.
Anyway, having calmed Lysa down, Petyr pushes her out of the door. I suppose that has got her out of the way. Petyr will probably try to marry Sansa next, and become lord of Winterfell as well as the Eyrie.
Oh, and he put the blame on Marillion. That guy had it coming, to be honest.
