To many, Sarah Jane Smith is the best Doctor Who companion of all time — smart, feisty, kind, and loyal to the Doctor, almost to a fault. Kind of weird, then, that the guy kicked her out of the TARDIS and left her on Earth years ago, never visiting her again. At least that's what School Reunion says happened (presumably erasing The Five Doctors with some kind of memory wipe). We'll go with it because it finally lets the series zero in on the Doctor's relationships with his closest companions, inevitably leading to a lot of feels: longing, regret, happiness, jealousy — this episode would make the Cybermen's heads explode. If you want to learn what makes Doctor Who tick, class is in session...

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Outro music: Chronos by Alexander Nakarada