However popular Mary is for now, she must still deal with an issue unique to female monarchs of the time.

-For a queen, if they marry, then their property becomes the heritable property of their husband, which immediately poses a huge issue for someone like Mary, because that is, in effect, her crown matrimonial -- the right of her family would pass to her husband.

And Elizabeth chooses not to marry, famously -- effectively becomes married to the realm of England.

And these are all different tactics that can be used.

So, it's a real issue for a queen.

If you marry, what does that mean for your own sovereign power?

-But a very tempting marriage opportunity arises for Mary: her cousin, Henry Stuart, Lord Darnley.

-Darnley looks good on paper because he is from a Scottish family, but has a line in the English succession.

-Together, Mary and Darnley have a very,very powerful dynastic claim to the English throne.

I think Mary is looking at him very clearly as, "You're what I need to get the English succession secured."

-He's young.

She thinks he might be manageable.

He is religiously ambiguous.

-And when she meets him as well, there is, of course, an element that's noted by a number of contemporaries of physical attraction.

-And she says he is "the best long man I have ever seen in my life."

Everyone fancied Darnley.

And she marries him, and I think they don't get out of bed for the first four days of their marriage.

-Yet, Mary's decision to marry Darnley is also a big risk.

-The marriage to Darnley is pursued without parliamentary approval in Scotland, and it's pursued without Elizabeth's approval, as well.

And that in itself leads to an alienation of the main council that had been in support of Mary through the first half of her reign in Scotland.

[ Thunder rumbling ] -Protestant lords mount a rebellion against the marriage... but the uprising is short-lived.

And initially, Mary's choice proves to be a smart one... in a crucial way.

[ Dramatic music plays ] ♪ -Mary gets pregnant.

It's a healthy pregnancy.

-She achieves what England's queens, Mary Tudor and Elizabeth I, never do.

But her husband is not what he seemed...