Doctor Who is that rare show in which anything is possible. Quite literally. With time travel, parallel universes, a boatload of alien creatures and a magic screwdriver, the show's writers can create any story, world, character or species they like, with very few limitations holding them back.
And while this freedom does mean that the Doctor Who universe's rules and continuity are often harder to figure out than a tangled pair of headphones (with so many writers injecting so many ideas, things were bound to get a little confusing in places) part of the fun in watching a show with such a rich history and lore is the theorising: piecing together seemingly unconnected strands, solving plot holes, and filling in blanks.
And with an infinite amount of ways for fans to explain any contrivances they may have with the show, or any unresolved stories and subplots it may throw out, it's no surprise that Whovians have taken to the web in droves to share their own wacky, creative, and (sometimes) brilliantly logical theories.
So grab your sonic screwdriver and hop inside your TARDIS, and let's head down the wibbly-wobbly online vortex of Doctor Who fan theories...