Every season of Doctor Who has an overall arc, whether it's "Bad Wolf" for season one, "Vote Saxon" for season three, or, most recently, the cracks in time for season five. Nobody knows where the cracks came from, but they have the ability to erase people straight from history as if they never existed!
Take for instance Rory, Amy Pond's boyfriend. The poor guy was not only shot to death by a Silurian in the denouement of "Cold Blood" (part two of the Silurian episodes), but he was sucked into the time cracks and was forgotten by everyone but the Doctor (because the Doctor's a complicated space-time event).
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