15x05 The Invisible Enemy Part One
This week's selection avoids the obvious choice of a Dalek story for something more conceptually linked: this is the story in which the Doctor is shrunk to enter his own body.
So I've read, anyway -- this is one I've not seen before (bonus!), and it doesn't happen in this episode. It's also the first story with K9... who's also not in this episode. Hey-ho.