Well that was just beautiful, heartbreaking and beautiful.

Finally a framing device/format change that really works for me, seeing the world through the eyes of a deteriorating K9, seeing Sarah Jane growing older as K9 only sees snapshots of her, hearing K9's assumption that he's been put in a box and forgotten.

> This unit is in a box.
> This unit has been put in a box and abandoned.
> This unit… has failed and been put in a box like old junk.

My heart.

But despite it all he still protects Sarah from being robbed, and he just about catches her calling him "A Girl's Best Friend" (love the reference).

And then the ending where you see inside K9's head at his thoughts on seeing The Doctor again.

Just so sweet, all of it