In the Tardisode, Mickey contacts Rose and the Doctor to help him investigate something strange goings on at a nearby school. Children are getting sick and there have been UFO sightings and other strange lights in the sky over the school

The episode opens with a great scene that many children will love. The Doctor decides to go undercover as a new teacher at the school to investigate, while Rose works in the school cafetaria. But to the Doctor's surprise someone else has decided to do some undercover work. Sarah Jane Smith, one of the Doctor's previous travelling companions.

Sarah doesn't recognise him at first but the fact that he introduced himself as John Smith started to get her thinking. The Doctor is so over the moon about meeting Sarah again after more than 200 years that his excitement is palpable. But he doesn't tell her just yet.

Sarah discovers the TARDIS in one of the rooms when she breaks into the school after dark and the Doctor is waiting for her when she comes out. The reunion is brief as they hear a terrible scream from...Mickey.

The headmaster is a member of the Krillitane race that absorb the best bits of races that they take over. They are really mansize bats but have taken the form of school teachers. They also like eating children. His team are using krillitane oil in the school cafetaria chips to increase the cleverness of the children so that they can crack the universal paradigm.

The Krillitanes are ready and the plan to take over the fundamental building blocks of the universe is almost ready to begin but not before some cat fighting between Sarah and Rose and some neat lines from Mickey. "Aah, the missus and the ex, welcome to every guy's worst nightmare."

Sarah confides in the Doctor that it has been so difficult to live knowing what she knows and not being able to tell anyone. Being dumped and never hearing from him, never knowing if he'll ever come back, meant she never really moved on with her life. (Trivia note: The location she was dumped at at the end of The Hand of Fear is finally revealed!) Contrary to popular rumour, she doesn't mention Torchwood, but Mickey's initial investigation is blocked by a Torchwood message.

After Rose and Sarah have it out they become the best of friends and work with the Doctor, Mickey and K9 to remove the Krillitane menace.

There was a slight hole in the plot that probably wasn't explained properly which is 'Why, if K9 isn't working, was Sarah carrying it around in her car?' Maybe she suspected that the Doctor would be there, or perhaps she's always kept him in the car.

Well, I've given enough of the plot away. There's still plenty of little touches that many fans will enjoy and I know this is going to be one of those ones that's watched more than once. Toby Whithouse' story is fantastic and the references to past Doctor Who adventures are accurate.

The ending to this story has been around for months and months so if you've read bulletin boards and other websites you probably already know the ending. So I won't go into details here. Suffice to say a major character dies and it's a tear jerker. If you cried during last year's Father's Day, you'll probably have a tear or two with this one. 9/10