In a not always subtle way, the fourth series of The Sarah Jane Adventures has explored the possibility of the titular character not having to always be on screen: in other words, the possibility that the show could survive in the hands of teen world saviours Rani and Clyde, who are certainly closer in age to the show’s target audience.

Now, our love for Elisabeth Sladen is deep, abiding, and has no equal, but it remains true that if the actress feels the need to move on, the show is now strong and established enough to continue on in her absence. Of course, we don’t want that to happen, and you don’t want that to happen (Sarah Jane Smith is Croydon’s finest ambassador, if only they knew), but it’s comforting to know that Team SJ have become a force to be reckoned with in their own right.

In this series closer, Sarah Jane comes up against a more confident, spikier version of herself – another gorgeous, brunette fifty-something who spends her spare time fighting aliens – basically, she’s the Call Of Duty version of Sarah Jane Smith (the internet’s femslash sites will likely have a few new stories before the week’s out). Rani and Clyde immediately warm to the new arrival, while Sarah Jane herself is beginning to feel a little old and unwanted.

It comes to something when the scariest moments (in a kid’s show, mind) are not those featuring a monster or a fleet of spaceships, but scenes in which Sarah Jane forgets a major character’s name – or, moments after commenting on her own age, spots her hands beginning to shake. In their own quiet way, these moments are more starkly terrifying than a fleet of Daleks or an infestation of Wirrin (ask your Dad). When we are told that a major character has an untreatable illness, it’s like a slap in the face.

There’s a great deal to enjoy about this story, including the appearance of a iPod version of Mr Smith and Rani driving a sports car (again, coming across as Sarah Jane Junior). A telling moment comes when Elisabeth Sladen’s character, attempting to comfort the young girl that there’s nothing wrong with her, catches her expression, and stops: ‘’No. I’m not even going to try’. Rani’s mum gets a very pivotal line, and there’s a moment of dissent between the two lead teens that, considering the slow burn deepening of their friendship over the last couple of months, is genuinely upsetting.

A two-part series finale of a sci-fi fantasy show aimed at kids could be about slimy creatures, aliens, and the end of the world. Certainly, if we were watching the ostensibly more mature Doctor Who, all of those would be a dead cert. That the main storyline is actually about a middle-aged woman coming to terms with whether or not she’s still relevant is pretty much astonishing. That there is very little sop to the idea that this is a children’s show is more astonishing still. Let us be clear here: this is still a show that’s aimed firmly at the seven year old in your household – it’s not being ransacked for the older audience. But it is a show that treats its young audience with respect, and (rightly) assumes that if the writing and performances are good enough, you don’t have to have green slime splattering the main characters. Although if you get that too, it’s a bonus.

Despite an awkwardly tacked on coda that looks and sounds suspiciously like somebody didn’t quite have the courage to let the final image speak for itself, and some occasionally overly simplistic dialogue (the very human concepts on show here are by necessity somewhat rushed) these two episodes are pretty close to perfection. This might be ‘Goodybe, Sarah Jane Smith’, but we’re obviously hoping it’s just Ta Ta For Now.

Airs at 5.15pm on Monday 15th November 2010 and Tuesday 16th November 2010 on CBBC.