29. School Reunion
One of the masterstrokes of the revived series was bringing back Elisabeth Sladen's Sarah Jane Smith in this lauded Series 2 adventure. Sarah was the definitive Classic companion for so many fans so to see her return in the modern series created the sense that the two were completely one and the same. Far from just aiming to please fans with a familiar face, though, School Reunion looks at the role of the companion like never before. What is it really like for a companion after they leave the Doctor behind, and just why does the Doctor pick up so many? The plot - Giles from Buffy gives schoolchildren brainy chips so that they can crack a powerful code - may be silly but the heart of the episode is superb. And it's always nice to see K-9. Affirmative!