George R.R. Martin's "A Song of Ice and Fire" series contains so many mysteries that are perfect for amateur sleuths to spend hours puzzling over in search of an answer. Sure, Jon Snow's death is a big question, but there's an even more important question surrounding his character. In "ASOIAF" parlance, the theory is known simply as "R+L = J," but it has massive implications for all of Westeros.
Jon Snow, even though he has grown up with the Stark children, is an illegitimate child. Ned Stark married Catelyn Stark, but the war started by his best friend, Robert Baratheon, led him south. During Robert's Rebellion, Ned had an affair that led to Jon Snow. At least, that's the story that has floated around after Ned Stark returned to Winterfell. Ned never gave a definitive answer, which only fueled the speculation over who was the mother of Jon Snow.
While Ned only managed to last one season on "Game of Thrones," it was enough to know that he was a noble man who valued honor to the point that it got him beheaded. It never made much sense that Ned would have an affair or betray his vow to Catelyn so soon after they were married. With the suspicion that Ned Stark was not the father, there were further clues found throughout the novels that led to the belief that the answer to Jon Snow's parentage could be found in the event that triggered Robert's Rebellion.
Lyanna Stark, Ned's sister, was betrothed to Robert Baratheon, but she was kidnapped by Rhaegar Targaryen, the son of the "Mad King" Aerys Targaryen. While the Targaryen reign was spiraling out of control, this event led to the war that dethroned Aerys Targaryen and put Robert Baratheon on the Iron Throne.
Rhaegar kept Lyanna at the Tower of Joy in Dorne before leading his army in the Battle of the Trident where he was killed by Robert Baratheon. As the war drew to a close, Ned Stark went south to get the last holdouts to pledge fealty to Robert. Ned arrived at the Tower of Joy to find three knights of the Kingsguard guarding the location. Among the Kingsguard was Ser Arthur Dayne, considered by many to be one of the greatest knights to ever live. A battle ensued where the only survivors were Ned Stark and his best friend Howland Reed. Inside the Tower, Ned found Lyanna dying in a bed of blood. Her dying wish was for Ned to promise to keep a secret.
That secret, according to the theory, was Jon Snow. That means his parents were Lyanna Stark and Rhaegar Targaryen, hence "R+L = J." Depending on who told the story, Rhaegar either kidnapped Lyanna and kept her prisoner or the two were actually in love and were trying to flee from their respective responsibilities.
That's a lot of backstory to tackle, but "Game of Thrones" Season 6 sees the return of Bran Stark, who has the ability to receive prophetic dreams, and set photos and trailers confirm the Tower of Joy will make an appearance.
It's unclear just how far the show will go with the "R+L = J" theory. Bran could use a Heart Tree to see a vision of a young Ned Stark battling to find his sister, where it's revealed she had a child prior to her death. This flashback could happen early in the season as a way to introduce Bran's abilities while setting up the return of Jon Snow from death.
Bran could also use his abilities to find his family, but discovers Jon Snow is dead. This could be another way to set up Bran searching the past for answers.