School Reunion is the third episode of Series 2. It featured the return of Liz Sladen as Sarah Jane Smith to the TV show, at the cost of her Big Finish solo series being cut short because Rusty was a cunt who wouldn't allow Briggs to use any characters that appeared in NuWho, even if they were originally classic in origin. He'd do this again with Davros later on, but I digress.
This story is widely renowned among the fandom as being a powerful gutpunch that reveals what happens to the Doctor's companions after he leaves them behind... Well, when I say "the fandom", I mean "mostly NuWho scrubs", because anyone who's seen Sarah's departure in The Hand of Fear and her first meeting with K-9 in K-9 and Company will know that she and the Doctor parted on good terms (especially good under those circumstances) and she held no grudge against him, so this sudden shift to her being cold and bitter because she was "dumped" is massively out of character for her, and shows that Toby Whithouse really didn't understand her at all. Still, Liz Sladen just about managed to sell it because she was a fucking phenomenal actress, but it's still a highly distracting plot point. Oh, and also Rose is a bitch for no reason.
On the plus side, K-9 shows up, looking disco as fuck.