Once you’ve had your horizons dramatically shifted, you’ll always come back different. Every one of the Doctor’s former companions knows this. They’ve seen things that the majority of their fellow humans don’t believe in. They’ve taken part in enormously significant events, but can’t ever really talk about it, because no one will believe them. They’re heroes and heroines, saviors of whole civilizations, but not on their home planet.

No one embodies this contradiction better than Donna Noble.

When it was first announced that Catherine Tate would become the Tenth Doctor’s next assistant after Martha Jones, people were concerned. Catherine was best known as a sketch comedienne, had already made a huge name for herself in the UK, to such a extent that she had managed to convince the Prime Minister, Tony Blair, to appear in a sketch with her.

Doctor Who has a thick seam of comedy running through it, always has had, but alarm bells were ringing. The show had only been back a few years, by bringing in a comedy performer with their own cultural baggage, could things tip over into panto territory? Was the show in danger of falling back into its bad old self-referential wink-wink “let’s put this in for the fans” ways?

Thankfully not. After two companions who clearly fancied the pants off of the handsome Tennenth Doctor (pun intended), Donna is the bucket of ice-water in his cocky face. There would be no “reversing the polarity of the neutron flow” with Donna Noble around, she’d be far more likely to look blankly at the Doctor and insist he just gets ON with it.

And Donna wasn’t just there to act as a latter-day Tegan. The insane cockiness of the Tenth Doctor needed tempering. After Martha had been transformed from a trainee doctor to a soldier, and her family traumatised by the Master, he needed someone who could help him take stock of where he’d been and the rules he chose to live by.

Donna’s role in the Doctor’s life was similar to that of an elder sister, looking after a child prodigy. Yes he can do stuff that she can’t do, but she ping-pongs between knowing this is astonishing and being appalled by what it all means. And she’s not shy of telling him either.

Here’s her first scene, another of the Tenth Doctor’s great “what? What? WHAT?” cliff-hangers:

And here’s the Doctor trying to cheer Donna up by bringing her into his world a bit:

And here’s a selection of Donna moments, funny and sad, sharp and soft. There are points during this selection when you could’ve renamed the show Donna Who and no one would’ve minded:

And when the comedy does come, it’s not silly or self-congratulatory, it comes from the fact that Donna is a right madam (and so is the Doctor). That familial bickering has been there throughout, which can cause some confusion:

And here’s Donna knowingly doing what every elder sister would do – the right thing – and trying not to get bitter about having to be the one to do it.

Oh and let’s have another look at her goodbye, shall we? Coming hard on the heels of the Doctor’s final farewell to Rose, it’s every bit as sad, but somehow crueller. Having saved the day, she gets no compensatory human Doctor to cuddle up to, no fond memories to look back on. She gave everything and got nothing back in return. Her horizons changed, and were then artificially put back again.

Tell you what, if she could remember what had happened to her, he’d be getting the telling-off of ALL. TIME, winning lottery ticket or no winning lottery ticket.

(You can order the two-part Tennant/Tate episode “Silence in the Library” and “Forest of the Dead” on the brand-new Facebook Video-on-Demand.)

Next: Martha Jones, the trainee doctor who became a soldier.

