Talk about giving a whole new meaning to the phrase, "You know nothing, Jon Snow." If this fan theory about Jon Snow's parents on Game of Thrones is true, Jon really knows nothing. About, like, his entire life. I would say that it would also void the hopes of Jon Snow/Daenerys Targaryen shippers (yep, that's a thing), but considering this show actually has one canon pairing that is ridiculously incestuous, those hopes can pretty safely remain valid.
So, what's the theory? Apparently, it's called the "R + L = J" theory, and it was, of course, originated by fans of George R. R. Martin's book series A Song of Ice & Fire, which is the source material of HBO's Game of Thrones. It suggests that Jon Snow isn't actually the bastard son of Ned Stark and an unidentified woman as we're led to believe (the official story is that Ned cheated on his wife Catelyn when he was away, and came home with a son) but that he's actually the illegitimate son of Ned's sister Lyanna Stark (married at the time to Robert Baratheon, Cersei Lannister's late husband), and Rhaegar Targaryen — who is, yes, be the son of the mad king Aerys II Targaryen and thus the brother of everyone's favorite badass mother of dragons, Daenerys Targaryen. Meaning, if this theory is true, Jon Snow would technically be Daenerys's nephew? Weird!
Anyway, if Jon Snow was indeed Lyanna's child, the secret would have really only been known by Ned and Lyanna. The theory suggests that when Ned found Lyanna in a "pool of blood" just before her death, the pool of blood was actually from childbirth, and Lyanna made Ned promise to raise Jon as his own in fear that King Baratheon would consider Jon a threat to the throne and have him killed. Thus, when Ned returned home, he returned with a child.
It's a crazy theory — and one that's basically unprovable for the time being, because the only people who would really know are Ned and Lyanna. Considering both of them can't really confirm or deny anything on account of being dead and all, it seems like this is something we'll all have to speculate about, at least for now. Luckily, Martin isn't finished writing the ASOIF book series, so it's possible there's still hope for his super elaborate theory.
Until then, check out a breakdown of it below and speculate to your heart's desire.