I'm starting my investigation with a very rare glimpse of Mary as a child.

Special collections.

In the stores at the National Portrait Gallery.

I'm hoping to be able to come face to face with the young princess.

Here are some exciting looking little boxes.

I'm here to see what's thought to be the earliest portrait miniature produced in England.

The art form's intended to give a sense of intimacy.

It's an image of Mary dating to 1522.

There she is, there she is.

Can I touch?

Yes Thank you.

You're very welcome, yeah.

It's Mary.

Incredible level of details.

She's got really red hair, hasn't she?

Yes, she does.

Like you'd expect from Henry the Eighth's daughter.

It's such a precious feeling, little thing and it's 500 years old.

Yes.

I'm interested in what this painting reveals about Mary's status.

The gallery's state of the art microscope might give me an even closer look.

It's just fantastic.

You can see the individual flakes of the paint.

When this portrait was made, Mary was a much loved six year old.

This was painted for a special reason.

And the clue to what that was... there it is, it's down here.

You can see that on her dress she's wearing a brooch, a golden brooch.

And it says on it in tiny letters, The Emperor.

So this is one of the European rulers.

It's the Emperor Charles the Fifth.

And the picture's been painted because Mary's just been engaged to him.

This is the fate of a princess.

She's like a little chess piece that her father is using to play the game of European politics.

The Mary I'm seeing here had a whole future mapped out.

But then in her teens, everything changed.

Here we have Henry the Eighth, and he's married to Catherine of Aragon from Spain, a very devout Catholic.

Poor Catherine had a whole series of miscarriages, stillbirths, children who died young.

Their daughter Mary, was the only one of their children to survive.

But Henry was desperate for a male heir.

He and Catherine had not had the all important son, so he wanted a divorce to marry Anne Boleyn.

In 1533, he got his way by splitting from Rome and the Catholic Church, opening the door to the English Protestant Reformation and dividing the country.

Mary was now declared illegitimate.

At 17, she was stripped of her royal title and threatened with death as a traitor for her beliefs.

She would come to define her life by her Catholic faith and her right to the throne.