2.3 - School Reunion

From IMDB:
The Krillitanes - aliens with a mix-and-match physiology - are trying to crack the 'God-Maker', a paradigm to give them ultimate power. They are using children as a computer, and only the Doctor and Rose, re-united with Sarah Jane Smith and K9 can prevent them from becoming masters of time and space.

1 out of 31 (3.2%) raters say this story requires a previous story.