Mickey calls the Doctor and Rose back to Earth to look into the strange problems at a school that is exhibiting peculiar goings on. The Doctor, posing as teacher, and Rose, as a cafeteria worker, seek to find out what the situation is. They are surprised when one of the Doctor's old companions, Sarah Jane Smith, and her robot dog, K-9, appear, to investigate the same peculiarities.
What they find is an alien race trying to use the young students to solve a code they sorely need to move up an evolutionary step.
The Doctor must defeat their attempt, while side stepping questions Sarah Jane Smith has about why the earlier incarnation of the Doctor abandoning her, to never, until now, be seen again. This causes Rose to wonder if this might be her future fate, too.