
So, in this post I'm going to explain the universe of Doctor Who in more detail if anyone is at all interested. ^_^
BASIS: It is about an alien who travels through time and space in his time machine/spaceship called the TARDIS. He mostly wants to explore and see the universe, but he always either gets mixed up into other people's problems or people ask for his help. Also, one of his companion points out "You never interfere in the affairs of other people or planets....unless there's children crying." Another one of his companions answers him when he asks "who am I to argue with history?" with "you're usually the first in line".

BASIS: It is about an alien who travels through time and space in his time machine/spaceship called the TARDIS. He mostly wants to explore and see the universe, but he always either gets mixed up into other people's problems or people ask for his help. Also, one of his companion points out "You never interfere in the affairs of other people or planets....unless there's children crying." Another one of his companions answers him when he asks "who am I to argue with history?" with "you're usually the first in line".
SERIES BACKGROUND: The show began in 1963 and is in the Guinness Book of World Records as the longest running Science Fiction TV show. It's also one of the first shows to use electronic music. The original series ran from 1963-1989. The "NewWho!" series picked up where the original series left off in 2005 and is still continuing now.
PLOT BACKGROUND: Both the original series and the new series follows the Doctor as he travels through time and space with his human companions exploring history and different worlds. However, in the new series the Doctor is the last of his kind. His planet was destroyed in a war along with all of his people (and the race they were battling), leaving him all alone. It is later revealed that he is the one that "pulled the trigger" so to speak and ended the war, which is why he was the only survivor. In the old series, the Time Lords sometimes gave the Doctor missions, or they punished him for breaking Time Lord law of non-intervention.
THE DOCTOR: The Doctor is the hero (and usually the protagonist) of the show. He is an alien called a Time Lord from the planet Gallifrey. He looks human, but he isn't. Although Time Lords share several human traits, they have some distinctive differences (ex: two hearts, the ability to regenerate [see below], etc.) The Doctor is not his real name, although his true name has never been revealed. There is some dark secret surrounding his real name and he is running away from it. He is often quirky and childlike, but he has a dark side that occasionally is unearthed. He is extremely brilliant in mathematics, history, science, medicine, technology, and so on. Basically, he's a genius. He is full of self-loathing in the new series because of what he did to his people. Because of his guilt, he never feels worthy of another creature's love, thus resulting in his obliviousness to his female companions. At the present moment, he is over 900 years old and is on his eleventh regeneration. The Doctor sums himself up in the episode Waters of Mars when asked to state his name, rank, and intention he answers: "The Doctor, doctor, fun." Also in The Eleventh Hour he says of himself: "I am definitely a madman with a box." One of his companions says regarding him: "What if you were really old and really kind and alone? Your whole race dead, no future, what couldn't you do then? If you were that old and that kind and the very last of your kind, you couldn't just stand there and watch children cry."
TARDIS: The TARDIS (Time And Relative Dimension In Space) is the Doctor's spaceship/time machine. It looks like a police box from the 1960's. This is because it has a cloaking device, so when the TARDIS lands it would disguise itself to blend in with the area. So if it landed in ancient Rome, it would disguise itself as a statue or something, but then at some point it landed in the 1960's, turned to a police box and the circuit got stuck. The Doctor liked the police box so much that he decided not to fix the circuit and keep it. The TARDIS is bigger on the inside. There's practically an entire world inside the little blue box. Apparently it needs six people to pilot it, but only the Doctor drives it (rather poorly according to River Song and Idris [the TARDIS in human form]). The Doctor threw the instruction manuel away because he disagreed with it. He stole the TARDIS from his own people when he was about 200 years old and ran away with it. The TARDIS is not necessarily a machine, it's alive. The Doctor doesn't even know the full power of the TARDIS.

REGENERATION: Regeneration is the Time Lord's trick. It's their way of cheating death. If a Time Lord is dying, but not yet dead, its body can repair itself if the Time Lord does not want to die. However, there is a drawback. The body repairs itself entirely, meaning every single cell is changed and rewritten, resulting in a new face and a slightly different personality. The process is "a bit dodgy" because they never know what they'll end up with. The Doctor has expressed concern about having two heads or becoming a girl. Also, each time he's regenerated, he always makes sure he has two legs and two arms first thing. A Time Lord does not have to regenerate, they have a choice between death and change. Also a Time Lord can still be killed if they are in the middle of regenerating or before they have a chance to start. Apparently it hurts to regenerate according to the Doctor when asked about it. The Doctor has said that regenerating still feels like dying because some new man "goes sauntering away" and his old self is dead. The Doctor has regenerated eleven times in his life, which is why the show has been able to last as long as it has.

The First Doctor was played by William Hartnell (age 55). He died simply from old age.

The Second Doctor was played by Patrick Troughton (age 46). He died because the Time Lords exiled him to Earth for breaking Time Lord law and forced his regeneration.

The Third Doctor was played by Jon Pertwee (age 50). He died from radiation poisoning.

The Fourth Doctor was played by Tom Baker (age 40). To date he is the actor to have played the Doctor the longest (seven years). He died by falling off of a radio telescope.

The Fifth Doctor was played by Peter Davison (age 29). He died by something called spectrox toxaemia poisoning.
The Sixth Doctor was played by Colin Baker (age 40). He died from injuries gained when the TARDIS crash landed after being attacked.
The Seventh Doctor was played by Sylvester McCoy (age 44). He died during a heart surgery because the doctors were not familiar with Time Lord anatomy.

The Eighth Doctor was played by Paul McGann (age 36). The cause of death is unknown since the series stopped and simply picked up with the Ninth Doctor. It probably had something to do with the Time War.

The Ninth Doctor was played by Christopher Eccleston (age 41). He died by absorbing the energy of the time vortex out of his companion, which she absorbed out of the heart of the TARDIS.
The Tenth Doctor was played by David Tennant (age 34). He is my favorite Doctor so far. He died by radiation poisoning.

The Eleventh Doctor is currently being played by Matt Smith (age 27). He is the youngest actor to have played to Doctor to date. No cause of death yet.
DOCTOR WHO: The show is called Doctor Who not because that's his name, but because that's what always comes after his introduction. --"Hello, I'm the Doctor." --"Doctor who?" --"Just the Doctor."
COMPANIONS: The Doctor loves all sorts of creatures, but he has a special fascination and love for humans. Therefore he spends a lot of his time exploring human history, landmarks, celebrities, and so on. On his travels he will often meet people (usually females) that he invites to travel with him. In the original series, it's usually just because. However, in the new series, it's still because of that, but also because he's lonely. Since his companions are usually females, lots of them tend to feel romantically towards him at times. However, he usually remains oblivious to this. Only twice has he actually fallen in love with his companions. Although he has kissed several of them, but for non-romantic purposes (or they kiss him and he reacts in a flustered and/or confused way. This is often for comical purposes).

SONIC SCREWDRIVER: The sonic screwdriver basically serves as the Doctor's weapon. He hates violence. He prefers negotiation and such rather than blasting aliens out of the air. He hardly ever carries a gun himself. Instead, he carries a sonic screwdriver. His own invention apparently. "What? You've never been bored? Never had a long night. Never had a lot of cabinets to put up?". It has a multi-purpose, but it is most frequently used as a lock pick. However it is also used as as a scanner, a tracker, and a remote control, as well as other things. Its only weakness is wood.
ENEMIES: Daleks are the Time Lords' mortal enemies. They first appeared in 1963 and are the oldest and most frequent reoccurrence on the show besides the Doctor and the TARDIS. They are they squid-like aliens that were mutated by a scientist to have no emotion other than hate. They are concealed inside a metal shell-like armour equipped with weapons. Their trademark is their (rather annoying) electronic monotone voice saying "exterminate!". The Daleks are who the Time Lords were fighting in the Time War. Both the Time Lords and Daleks were apparently wiped out. However, the Daleks always managed to somehow survive by falling through the vortex or something and thus coming back to exterminate the last Time Lord.

The Master is another Time Lord. He is the "Moriarty to the Doctor's Holmes". Apparently, he and the Doctor were childhood friends. He is a lot like the Doctor, but he is completely insane. The Doctor explains that the Master's madness originated from his childhood. Time Lords, at the age of eight, were to look into the time vortex as some sort of initiation. Some would be inspired, some would run away, and some would go mad. The Master was one that went mad. The Master is a wanna-be universal conqueror, thus making him a great threat to the Doctor who wants peace in the universe. The Master, being a Time Lord, has regenerated several times in his life, but I'm not quite sure how many since he's had a rather complicated life (full of resurrection, possessing other's bodies, and running out of regenerations, but still being able to live somehow). He has actually died twice, but has been has been resurrected both times. Once by the Time Lords to help fight in the Time War, and again by humans for a rather vague reason.

STORY ARCS: Each season has its own story arc. The new series has been especially heavy on story arcs. A story arc is an extended story which takes place over a large number of episodes or serials, characterised by a gradual unfolding of plot points and introduction of narrative devices which come together in the arc's climax. Some examples of story arcs: Bad Wolf, Torchwood, Mr. Saxon, Medusa Cascade, "he will knock four times", cracks in the universe, and "silence will fall when the question is asked".
The show is not all adventure and seriousness. There's loads of comedy and quirkiness too. I often find myself laughing my head off sometimes. The Doctor especially is just full of quirks and childishness. And of course there are some upright terrifying episodes that give me nightmares for eternity. (Angel statues, double shadows, gas masks, etc). One thing I absolutely love about the show is that it is so full of optimism. Not everything always goes perfectly and the characters will make mistakes, but there is always hope. Some awesome quotes come out of this show.
There is sooooooo much more to this show. I could go on about the different companions, the time vortex, the TARDIS, his sonic screwdriver, time streams, the Doctor meeting his past incarnations, different aliens and planets, expounding on the story arcs and the complexity of the Doctor himself. But if I did that, this post would literally turn into a novel. I'm just going to leave this at... Doctor Who is a freakin' AMAZING show!!!
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The Tenth Doctor accidentally crosses his own time stream and meets his past self, the Fifth Doctor