10 Saddest Doctor Who Companion Exits
7. K9 MKIII in School Reunion
Of the many innovations of the Russell T Davies era, one of the lesser-championed ones was how it made audiences weep for K9. In the late 70s and 80s, writers would regularly blow up the tin dog, or contrive ways to keep him in the TARDIS in order to avoid writing for the character. In Toby Whithouse's School Reunion, however, K9 blowing up holds dramatic weight.
Staying behind to defeat the Krillitane, K9 detonates the oil and perishes in the explosion. He even gets to deliver an action hero style one-liner! As the kids all celebrate their school being blown up, the Tenth Doctor puts his arm around a grieving Sarah Jane. K9 MK III's death is so affecting because he has come to represent Sarah's last connection to her life with the Doctor. He hasn't worked properly for years, but she still keeps him in the boot of her car, almost as a memento.
When he dies, it severs that link with her old life. It's heart breaking, but it gives Sarah the chance to move on and form a new relationship with the Tenth Doctor, Rose and Mickey. And even though she gets a shiny new K9, it doesn't undermine the noble sacrifice of K9 MK III. Good dog.