Okay, I'm a newbie to Dr Who and I'm slightly confused on a few things with the second season.

I know that Dr. Who is known for going through Doctors than Spinal Tap has drummers, but how'd they explain the new doctor's appearance from season 1 to season 2?

I haven't seen the second half of season 1 yet, but I did see the season two opener. They brought up the "new new doctor" 's look a few times, but there wasn't much of an explanation. Did I miss something in the season 1 finale? Or is this just a wry joke about how the original series switched actors so fluidly?
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