Spoilers for Game of Thrones, season 7 below.
Winter has arrived on Game of Thrones, and Sansa and Arya Stark's reunion at Winterfell is turning icy pretty fast. There's already conflict between them, and now sneaky Littlefinger is trying to pry the rift open further.
At one point during "Eastwatch," Littlefinger procures a note from Maester Luwin’s extensive archives. From the way he's sneaking around the castle, it seems pretty clear that he doesn't want anyone to know that he has it.
So then Petyr Baelish goes and hides the note, but (surprise!) Arya is determined to find out what it says. And after a quick search of Littlefinger's chambers, she finds it, hidden in his mattress.
So what does it say? A Redditor transcribed the whole thing.
Robb, I write to you with a heavy heart. Our good king Robert is dead, killed from wounds he took in a boar hunt. Father has been charged with treason. He conspired with Robert’s brothers against my beloved Joffrey and tried to steal his throne. The Lannisters are treating me very well and provide me with every comfort. I beg you: come to King’s Landing, swear fealty to King Joffrey and prevent any strife between the great houses of Lannister and Stark. Your faithful sister, Sansa
This is a note from season one, when Cersei Lannister made a young Sansa Stark plead with her brother Robb to bend the knee to the new King Joffrey. At the time, their mother Catelyn Stark realized that the note was being written under the watchful (and forceful) eye of Cersei Lannister, and she advises that they ignore the contents of the letter and focus on saving Sansa.
Arya may not agree with her late (or is she??) mother's assessment of the note. If she thinks that Sansa betrayed her family by siding with the Lannisters in King's Landing, it might make her even more pissed off at Sansa—with whom tensions were already boiling up—which can only mean bad things in the coming episodes for House Stark.
That is, unless Arya knows that Littlefinger planted the note for her to find.
Oh, and I guess that marriage between Sansa and Dickon probably isn't going to happen. Still, it was a cool idea, though.