The Doctor masquerades as a teacher (and Rose as a dinner lady!) to try and uncover some weird goings on in a school. They meet some old friends and some old monsters, which provides a good story on two levels.
SPOILERS AHEAD
It's the one with K9 and Sarah Jane! 
Despite some great acting as a menacing headmaster by Anthony Head (who bizzarely is now known in the UK more for his role in Little Britain than Buffy) the monster story is a mere vehicle for the reintroduction of K9 and Sarah Jane. It was a nice story, Sarah Jane explaining how she was left by the Doctor, whilst she was clearly in love with him. And Rose was immensley jealous of Sarah Jane showing up, proving that her relationship with the Doctor is more than just travelling companions. The Doctor seemed to keep out of it until the end, despite being clearly thrilled to see Sarah Jane. Finally he explained how it was terrible seeing his companions grow old, very Highlander. David Tennant showed the emotional side of the Doctor nicely and Billie was good as the jealous girl.
It was also a big episode for Mickey, who did his usual run-around and save the day type action, but also realised that he was the modern equivalent of K9, the robot dog. And then getting an invite at the end to join the Doctor and Rose. I loved Rose's expression when the Doctor said yes, real anger. And I loved how Rose asked Sarah Jane whether it was worth staying with the Doctor and she replied that some things were worth getting a broken heart for.
K9 was highly amusing, with his silly little voice, rusting shell, tartan collar and laser in his mouth. When Mickey and Rose saw him for the first time they remarked how old he looked, showing heavy sceptcism, at which the Doctor was offended.
So I think it was two episodes here, the action and monsters for the kids, and the higher emotional story for the adults. I enjoyed it and I think it worked well.
Here's the online stuff and a wikipedia entry.
