There was one key message Kate wanted us all to see from her touching cancer update video

Her message was plain and unvarnished – the past nine months have been a terrible ordeal. But her family – and the nation - gave her the strength to carry on.

This breathtakingly intimate film laid bare what the power of love can do to help carry a cancer sufferer through the dark days and long nights. And as Catherine wanted us to know, her journey isn’t over yet: “My path to healing and full recovery is long,” she said.

“I take each day as it comes.”

The words were reassuring, softly spoken. The images which accompanied them reinforced her message but went much further – opening a new narrative on Catherine and William’s family life, giving the world an unprecedented insight into their loving relationship.

Never before has there been a film which shed so much light on a royal couple’s private life, and it served as a reminder that Catherine – who we’ve missed so much – has a colossal dual role in life, as a princess and fashion icon, but also as a wife and mother.

The focus on her, since her marriage to William in 2011 has been intense, but rather than beg for more time to recover and present an image to the world of health and beauty – as we’ve come to expect from a Princess of Wales – she promises she’ll be back with us, undertaking more engagements “when I can”.

That phrase is just a reminder of the fragile world she’s inhabited these past nine months since it was revealed she was suffering from cancer.

But, as she so eloquently put it, “Out of darkness can come light – let that light shine bright.”

And the light, it was plain to see from her film, comes from William, George, Charlotte and Louis. Their walks together through the woods, on the Norfolk sands, and at home with grandparents Carole and Michael Middleton were heartwarming. Clearly these two have played a much greater part in the lives of their granschildren, struggling to cope with their mother’s illness, than has ever been revealed.

But the best moment was saved for when Catherine put her head on William’s shoulder – a telling intimacy they might never have shared with the rest of the world has she not suffered so terribly, and been so relieved to have come through the dark days .

“I remain with you – side by side, hand in hand,” she said, a heartwarming phrase which referred equally to those who closely surround her, but also the nation, who she went out of her way to thank for the kindness, concern, and support she’s received through the long months of illness and slow recovery.

She looks well, but understandably the ordeal Catherine has been through has visibly left its mark, and we cannot expect too much from her too soon. “The last nine months have been incredibly tough for us as a family – we’ve had to find a way to navigate the stormy waters,” she explains, a signal perhaps that the waters may not yet be calmed as she fights her way back to full health.

Why Catherine and William have chosen this moment to reveal so much as they have in this deftly-produced film – intriguingly halfway between home movie and a Hollywood epic – is difficult to assess. Maybe they feel that they’ve been silent too long, and that people rightly have an expectation to know how she is, whether she will be able to return to the world stage.

Equally, it may go some way towards explaining – without saying it in so many words – why William’s public appearances have been strictly rationed, at a time when the royal family has never been more debated and discussed.

With King Charles still in recovery from his own cancer, some critics might have argued that, currently, the royals aren’t giving value for money. But a glimpse of Catherine as she wandered through the woods with her husband and children, elegant but demonstrably still frail, quickly dispels such ideas – royals are human beings too, subject just as the rest of us are to unexpected crisis.

The family Catherine and William present to us in this film is a strong one, bound by love and affection, by pride and determination.

In the film, the princess thanks everyone again for their love and compassion. Just to let you know, Your Royal Highness, we have plenty more of that to give.

You are very precious to us.