One of the interesting ways in which new Doctor Who has developed from the classic series is the emphasis on how much of the Doctor's behavior relates to his relationship with his companion, particularly the first companion for each incarnation. During the classic series the companion was generally someone the Doctor enjoyed traveling with and of which was deeply fond, but in the new series the function of the companion has grown into a partner who provides this alien time-traveler an absolutely essential measure of humanity and perspective.
This in turn has tremendously increased the emotional, if Platonic, bond between the Doctor and his companion.
This is particularly true for the first companion a new series Doctor takes on, and that even more so over time. From the beginning, "Rose" -- the pilot, introducing both the Ninth Doctor (Christopher Eccleston) and Rose (Billie Piper) -- was all about her, not him, and it ends with the Doctor asking more than once for Rose to join him on the Ship.
By the time we get to "The Eleventh Hour," the introduction of the Eleventh Doctor (Matt Smith) and Amy Pond (Karen Gillan), the function envisaged for the companion has expanded nearly to the point of equal partnership, with considerable emphasis on the Doctor's potent connection with Amy and her unnatural capacity to make sense of certain aspects of his universe (thanks to a time-rift crack in her childhood bedroom wall).
Which makes the transition to the Eleventh Doctor's first new companion high stakes indeed.
Replacing Your First
We've already seen the prototype: in season 3, when the Tenth Doctor took on young medic Martha Smith after being forced to let go of Rose, poor Martha spent most of the season being overshadowed by Rose's invisible presence. At one point he muses about what to do and, looking right at Martha without seeing her, laments that Rose would've known what to do.
It was shame for Martha, who made a smashing companion and who ended up leaving mainly because her relationship with the Doctor was so conflicted, and even to a certain extent for the brilliant Freema Agyeman who played her.
And the Eleventh Doctor is far more interdependent on Amy that the Tenth was with Rose: after all, Rose and the Tenth were only together for a season, but Amy and the Eleventh, on top of their deep and instant bond on first meeting, have been together for two and a half.
(Amy and Rory will leave and/or die -- Steven Moffat has been warning that not everyone will get out alive "and I mean it" -- in episode 5, just before the season halfway break. Yes, season 7 will be split in two, with the second half inaugurating the 50th anniversary year of 2013.)
The New Companion
So what troubles are in store for Jenna-Louise Coleman, just announced as the new companion? So far we don't know her character's name or background, though details will be announced very soon; just that the Eleventh will be in a state after separating from Amy, however traumatic or tragic the break is.
"It's not often the Doctor meets someone who can talk even faster than he does, but it's about to happen," said Moffat. "Jenna is going to lead him his merriest dance yet. And that's all you're getting for now. Who she's playing, how the Doctor meets her, and even where he finds her, are all part of one of the biggest mysteries the Time Lord ever encounters. Even by the Doctor's standards, this isn't your usual boy meets girl."
"It always seems impossible when you start casting these parts," continued Moffat. "But when we saw Matt and Jenna together we knew we had our girl, She's funny and clever and exactly mad enough to step on board the TARDIS."
Coleman added "I'm beyond excited, I can't wait to get cracking; working alongside Matt I know is going to be enormous fun and a huge adventure."
Not Completely Innocent
The shift will be jarring for the Doctor, but the real question is how tough it will be for the fans. At least Coleman is no novice, or stranger to nonconformity: she rose to fame as teen lesbian Jasmine Thomas in the "northern" UK soap Emmerdale, and was also a fixture on Waterloo Road.
Those who haven't seen either of those might have noticed her -- though it's not likely -- in the small role of Connie, Bucky's date (the brunette) as he tries to interest Steve in a double date at the future technologies exhibition, in Captain America (she can be seen here).
One more thing to bear in mind. The Eleventh Doctor has been traveling with Amy and Rory -- even before Rory joined the TARDIS crew officially he was mentally and often physically present. So the transition from old to new might very well be aided by the different dynamic brought about by a reversion to the Doctor-plus-one format.