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There are many "Game of Thrones" fan theories out there, but arguably the most popular is the one behind the true parents of Jon Snow.
In "Game of Thrones," Jon Snow (who is played by Kit Harington on the HBO series) is known as the bastard child of the Lord of Winterfell, Ned Stark.
However, since the story constantly points out Jon's conflict with his illegitimacy and Ned's strict code of honor, many fans have come up with a theory that actually explains who they think Jon's parents really are.
Fans know the theory as "R+L=J." "R+L=J" would refer to Jon Snow ("J") being the child of Rhaegar Targaryen ("R") and Ned Stark's sister, Lyanna Stark ("L") rather than Jon being the illegitimate child of Ned.
Considering this is one of the best kept secrets in the entire series (so much so that the creators of the HBO show were quizzed on it by author George R.R. Martin himself), YouTube user "Alt Shift X" put together a video explaining the theory and pulling out facts that help prove it.
The theory explains that Ned's sister Lyanna was set to marry Robert Baratheon when she "stolen" by Rhaegar Targaryen who brought her to the Tower of Joy in Dorne. This act begins the war that later sets "Game of Thrones" into action.
However, the theory makes the argument Lyanna was actually in love with Rhaegar and ran away with him.
After Robert kills Rhaegar thus ending the war, Ned Stark heads to the tower and finds his sister in a "bed of blood."
With her dying breath, Lyanna forces Ned to make her a promise.
No one knows what this promise was, but many theorize that Lyanna died during child birth (hence the "bed of blood") and asked Ned to pretend her child Jon was his.
This was done in order to protect Jon from Robert and the Lannisters, who were killing all of the Targaryen children via Ser Gregor "The Mountain" Clegane.
Now, there's plenty of evidence hidden throughout the story that shows that Jon isn't Ned's son out of wedlock.
For example, a quote in the first book shows how Ned is an incredibly honorable man who keeps his vows including those to his wife, Catelyn.
Another piece of evidence is that when Daenerys (who would be Jon's aunt in this theory) was in the House of the Undying she saw a blue flower growing out of a wall of ice.
Blue flowers are used throughout the series to represent Lyanna while Jon is a member of the Night's Watch stationed on the giant ice block that is The Wall.
Now, no one really knows if the theory is true or not, but if it is then this would possibly mean that Jon has a better claim to the Iron Throne that anyone else in the story, including Daenerys.
It would also mean that Jon Snow would represent both ice (Stark) and fire (Targaryen) which the entire book series was named after ("The Song of Ice and Fire").
To get the entire break down of the theory, check out the full video below: