It's always nice to see companions return and School Reunion sees the return of not one but two of them. School Reunion reintroduced Sarah Jane and K9, both reprised by their original actors Elisabeth Sladen and John Leeson and effectively acting as a backdoor pilot to the spinoff The Sarah Jane Adventures.

It's bittersweet to see the return of such beloved companions, especially when the excitement at seeing a familiar face again is played so well by David Tennant. Toby Whithouse writes the reunion so well that you can't help but feel a little disappointed that she decides not to join the Doctor on his adventures again.

K9's emotional sacrifice is touching and fortunately the introduction of a K9 Mark 4 doesn't ruin it but instead allows for another sweet moment in the friendship of the Doctor and Sarah, where the TARDIS dematerialises to reveal the new K9.

But what of the actual plot? Well, School Reunion sees the Doctor and Rose investigate Deffry Vale School after Mickey (Noel Clarke) tells them over the phone about some strange goings-on. There, they find that a race known as the Krillitanes have taken on the form of teachers at the school and the canteen are serving chips in Krillitane oil to increase the kids' intelligence so they can help crack the Skasis Paradigm and control the universe.

The story can feel a little contrived considering the Doctor and Sarah Jane just happen to be investigating the same school at the same time but it is forgivable given how great everything else is.

The Krillitanes are a fantastic concept for an alien species; these are aliens who take on the features of any species they conquer meaning they could look different each time they return. And they need to return. In fact: why haven't they yet? This is such a strong concept that with return appearances they could arguably become among the most iconic of new series monsters.

Anthony Head deserves to return as Mr Finch also; he is scarily cold and menacing as the school headteacher, especially during his tense confrontation with the Doctor at the swimming pool (I've never heard of a school with a swimming pool before!).

It's nice to see Mickey Smith finally become a companion at the end of this story. I always liked Mickey Smith, right from his introduction in Rose up until his exit in Rise of the Cybermen/Age of Steel, and subsequent returns. He should have been a companion sooner and indeed when he is, the chemistry works really well between the Doctor, Rose and Mickey. School Reunion is followed by another great story,.The Girl In The Fireplace, and one which I will review at a later time.

Overall, School Reunion is a fantastic reunion between three characters (the Doctor, Sarah Jane and K9) even if it does feel a little contrived and introduces a new race with a clever concept who really should return.