15x06 The Invisible Enemy Part Two
There's K9, and the miniaturisation! This isn't a very well-regarded story, and it's not all that, it's true. Has its moments though, most of them courtesy of Leela's humour.
1x08 The Challenge
1x11 (1/9/14 edition)
1x12 (2/9/14 edition)
[Watch episodes 11 and 12 (again) on iPlayer.]