How Gilly’s discovery in last night’s Game of Thrones could derail the entire show

It's pretty terrible news for Daenerys

Here's why it's never a good idea to nip away during Game of Thrones to make a cup of tea: massive, series-changing plot twists can be tossed into the mix when you're least expecting them. Case in point was last night's accidental discovery by Gilly while she was sitting with Sam in the Citadel library.

Had Sam been listening (eye roll) he would have been party to a major upset in the line of Westerosi succession. But instead, he interrupted her mid-sentence to talk about something else. Gilly, in this you have the sympathy of women the world over.

So what exactly did Gilly discover and why does it matter so much?

Monday night's episode shows Gilly sitting in the library opposite Sam (who is in a foul mood after watching a plea from Bran to help the North against the White Walkers get batted away by the senior maesters), reading a set of records written by a High Septon.

'This High Septon Maynard, he recorded everything' Gilly remarks to Sam. 'He even recorded his own bowel movements. Maynard says here that he issued an annulment for a Prince ‘Ragger’ and remarried him to someone else at the same time in a secret ceremony in Dorne. Is that a common thing in the South or…?'

Unfortunately Sam is too lost in his own thoughts to notice this MASSIVE PLOT BOMBSHELL that's just been tossed into his lap.

Let's break down what this means and why it's so important. So we already know, thanks to the now-proved Game of Thrones fan theory (R+L = J) that Jon Snow and Daenerys are related. Previously passed off as the bastard son of Ned Stark, we discovered last season that Jon Snow's father is actually Rhaegar Targaryen, the oldest son of Aerys (the 'Mad King'), and his mother is Ned Stark's sister Lyanna. Meanwhile, Daenerys is the younger sister of Rhaegar, making her Jon Snow's aunt.

But this is all stuff we knew before. What we didn't know until last night is that according to this High Septon's records, Prince Rheagar - who was married to Elia Sand - secretly annulled this union and remarried 'someone else' at 'a secret ceremony in Dorne.'

What does this mean? We had previously assumed Jon Snow was the illegitimate child of Rhaegar Targaryen and Lyanna Stark - making him a blood relative of Dany's - but if these Septon's records are correct (and the 'someone else' is indeed Lyanna) then Jon Snow is a legitimate male Targaryen, which in Westerosi terms is huge: as the grandson of King Aerys, Jon Snow - not Dany - is the rightful heir to the Iron Throne.

And Sam would now know all this about his best friend if he'd been listening. But he wasn't.

Before Gilly's inadvertent discovery, Jon's status as a legitimate Targaryen was hinted at when Dany returned from battle and landed on the mountainside on Drogon's back. Looking terrified, Jon still approached the dragon and slowly put his hand on its snout. Anyone other than a Targaryen would have been blowtorched before he'd got even close, but the dragon seemed pretty chilled about it - something Daenerys appeared to clock.

It can't be a coincidence that the same episode we saw Jon's claim to the throne legitimised, we watched Tyrion and Varys's relationship with Daenerys come under strain when she allowed Drogon to torch Randall and Dickon Tarly to death in front of a group of Lannister army captives after they refused to bend the knee.

Does this hint at a future breakdown of Dany's support squad? Will they switch their allegiance to Jon, particularly once they find out that he's the legitimate heir? And, even more importantly, how will Daenerys react after spending her life labouring under the belief that she is the only rightful heir to the Iron Throne?

Another very feasible future plot development that could solve the power struggle: a Dany and Jon marriage. She definitely looked sad to see him go off to Eastwatch. She also seemed to enjoy watching him stroke her dragon (I meant that literally, don't be filthy).

As ever with this show, expect the unexpected.