6. The Doctor's Wife 6.4
There were some episodes that I knew the moment I had decided to write this article that needed unequivocally to be here. The Doctor's Wife is one such episode. There are a number of interesting elements in this story: The idea of the House, a monster that is itself a planet and also a semi-transferrable energy; Auntie and Uncle two humanoid cratures made from Time Lord spare parts; introducing the idea that renegade Time Lords travelling in TARDISes were far more common than had previously been implied; and the cameo appearance of an Ood, my personal favourite Doctor Who alien. All that aside, there is only one true reason it makes a list of pivotal story arcs. Human TARDIS. When Idris takes on the mind of the TARDIS allowing the Doctor, and companions Amy Pond and Rory Williams, to talk directly to her, it furthers the Doctor TARDIS relationship more than ever before. It proves definitively that the TARDIS is not only living, and displaying a consciousness, as has been suggested for some time, but also has a will and personality. It also finally offers a rationale for why she so often lands them in the "wrong" spot.
5. Dalek Invasion Of Earth 2.4-2.9
No truly historical list could skip this serial, perhaps the most well-known of all of the original Doctor's adventures. This grim four-parter is most important for being the first ever companion departure. Here we see the Doctor say goodbye to his granddaughter Susan, setting him up for the pattern of inevitable goodbyes that will follow him for the rest of his lives. Additionally, this serial is responsible for establishing a lot of the horror factor behind the Daleks. There is this idea introduced of Dalek human hybrids introduced called Robos, who "go mad" once the Daleks are finished using their transplantation-altered minds. Perhaps an early test drive of the Cybermen, this lethal fate was to have insane Robos dashing at their brains, unable to handle the atrocities they had committed while exterminating other humans. The story is surrounded by death, the scale of which we have not seen before. We can understand why Susan stays behind with a local man, David Campbell, to continue fighting the fight still left to be waged there, and to rebuild.