20. The Time Warrior 11.1-11.4
This is the first serial to feature companion Sarah Jane Smith. Sarah Jane establishes herself a different sort of companion, especially for this Doctor. While his other two companions Liz and Jo are thrust upon him and he upon them, Sarah Jane deliberately signs on, even purposefully stowing away abroad the TARDIS. Equally, if not more important, this is the first story to refer to Gallifrey by its name. Up until now it is simply the Time Lords' homeworld. This story is also the first to feature long time villains, the Sontarans. In addition to being visibly physically different than Sontarons of the Modern Who erathese Sontarans are both taller and less sleek than say, Strax they also differ in strategy. While all Sontarans are warriors, the Sontaran fleet depicted here displays a peculiar sort of cunning manipulating humans into rounding out their battle forces and acting toward their objectives.
19. Fury from the Deep 5.29-5.34
This long recon serial featuring The Doctor, Jamie and Victoria Waterfield is most notable for being the inaugural appearance of the sonic screwdriver. This time the screwdriver performs the function for which it is named, removing the screws from a mysterious box discovered by the Doctor and company, the Doctor investigates the box's contents using a stethoscopesomething reprised by several other Doctors, especially the Tenth Doctor. This same scene features a foam fight which again highlights this Doctor's active silly side. The scene concludes with our team of time travellers tranquillised and kidnapped abroad a struggling fuel refinery, which as per usual, is plagued by both greedy and somewhat corrupt humans and a monster problem. After such an eventful start, much of the next several parts is just more of procedure, sociopolitical drama with the humans, danger for our trio, and discoveries about the seaweed and foam monster that causing the trouble this time. We see Victoria voice her objection to travel. But things really pick up in part six when a combination of Victoria, the Doctor's ingenuity, and sound itself work together to save the day, which just maybe served to cement the screwdriver's place in history.