Basically, the series revolves around the Doctor, a member of an near-human alien species known as the time lords, who have mastered time and space travel. However, they choose to sit back and observe the universe, only interfering when absolutely necessary. The Doctor however grew sick of time lord society, and rebelled; he stole a time machine or TARDIS (Time and Relative Dimensions in space) and decided to go on adventures throughout all of time and space, fighting injustice and sometimes just trying to enjoy himself.
At one point, his time machine's Chameleon circuit-which allows the TARDIS to blend in with it's surroundings-jammed and now it's been stuck in the shape of a Police box-on the outside at least. Inside the TARDIS is a massive spaceship with near-infinite rooms and corridors, although the 'bridge' of the TARDIS is the console room, which houses the ship's main controls, enterance and viewscreen.
The Doctor is frequently accompanied by companions, who are usually humans, although there have been other aliens and robots among the 'crew'. He has a big sense of humor, although what type of humor changes with the incarnation.
Time Lords have the ability to 'regenerate' when near death, changing into a new body. Although the mind and memories of the Doctor remain the same, there are often radical changes in his personality and methods. After his regeneration the Doctor also often experiences temporary amnesia and psychosis-this is mostly due to the Doctor's regeneration's being more traumatic than most other time lords (Who pretty much just die when they get old).
Here's a general rundown of the Doc's incarnations.
There are some spoilers ahead. I've included some quotes, as well
The First Doctor (William Hartnell) 1963-1966
"One day, I shall come back; yes, I shall come back. Until then there must be no regrets, no tears, no anxieties. Just go forward in all your beliefs, and prove to me that I am not mistaken in mine."
When we first meet the Doctor, he's an apparent exile from some strange civilization. He appears somewhat sinister at first, but then sort of adopts a kind-if stern-attitude. He also had a mischevious sense of humor.
In his youngest incarnation, the Doctor fought the Daleks and Cybermen for the first time and crossed paths with the Aztecs, Marco Polo, King Richard and other historical figures. His era ended when Hartnell had declining health problems, and so the concept of regeneration was brought into the show. Collapsing after a battle with the Cybermen, Hartnell regenerated into:
"There are some corners of the universe that have bred the most terrible things. Things which act against everything we believe in. They must be fought."
The Second Doctor (Patrick Troughton) 1966-1969
This Doctor was much younger-looking and energetic than his predecessor. A sort of 'galactic hobo', this Doctor was more humerous than his predecessor, although his clownish antics hid often an incredible intelligence and a slightly more manipulative nature. This Doctor fought the Daleks and Cybermen multiple times and also was the first to encounter the Yeti, Ice Warriors, and UNIT. At this point, the Doctor's past caught up to him and he was captured by his own people, the time lords. Putting him on trial for taking a time machine and interfering in the universe, they forced him to be exiled to Earth and regenerated into:
The Third Doctor (Jon Pertwee) 1970-1974
"You know, sometimes I think military intelligence is a contradiction in terms!"
This Doctor, who had the appearence of a middle-aged man, became military advisor to UNIT, a military organization that dealt with strange occurences. Mostly, he helped repel alien menaces bent on invasion as well as a rival time lord known as the Master. He also made the ocassional trip in the TARDIS, but not as much as his other incarnations.
Personality-wise, he was very much an authority figure, more serious than his previous incarnation but also more flamboyant, with frilled clothing and a large variety of gadgets, including two customized cars (One of which could fly). He was very much a man of action, often using karate to subdue the bad guys. After a run-in with a dangerously radioactive cave, the Doctor regenerated into:
The Fourth Doctor (Tom Baker) 1974-1981
"The trouble with computers, of course, is that they're very sophisticated idiots..."
The longest-running Doctor, this Doctor was perhaps the most eccentric of the incarnations, wearing a long scarf and having a taste for sweets. Breaking most of his contact with UNIT (Although ocassionally helping them out), he resumed his travels in time and space. Like the second Doctor, his goofball persona hid a vast intelligence, and the fourth Doctor was also very contemplative of things, which led to him brooding from time to time.
In this incarnation the Doctor first encountered the creator of the Daleks, Davros, a mad scientist who would continue to haunt the Doctor in later incarnations. He also encountered memorable one-time villains such as the Zygons and Sutekh.
Toward the end of his reign, the Master returned and nearly destroyed the universe. In an attempt to repair things, the Doctor was forced to work with the Master, but the Master betrayed him, causing the Doctor to plummet off a radar tower. He regenerated into:
The Fifth Doctor (Peter Davison) 1981-1984
"An apple a day keeps the... Ah, never mind."
This Doctor, although older than his previous incarnations, appeared as a man apparentally in his early thirties. Although one of the kindest and selfless of the Doctors, he did have a somewhat short temper, especially when it came to his often-complaining companions.
He was also very energetic, often bursting into runs when danger was ahead. Unfortunately, his nature often let villains easily take advantage of him, especially when his companions were threathened. It was for a companion that this Doctor gave his life-after contracting an illness on the planet Androzani minor, he only had enough antidote for one person. He gave it to his companion, Peri, and changed into:
The Sixth Doctor (1984-1986) Colin Baker
"I am the Doctor, whether you like it or not."
The Sixth Doctor was loud, arrogant, and at times borderline psychotic, choosing to wear a wacky patchwork coat. However, beneath all his gruff exterior there was a very compassionate and poetic man who cared deeply for his companions.
However, slipping ratings, an eighteen month hiatus and behind-the-scenes drama at the BBC cut this Doctor's reign short, although the actor Colin Baker showed a great enthusiasm for the role. Without much explanation, he regenerated into:
The Seventh Doctor (Sylvestor McCoy) 1987-1989; 1996
"There are worlds out there where the sky is burning, and the sea's asleep, and the rivers dream; people made of smoke and cities made of song. Somewhere there's danger, somewhere there's injustice, somewhere else the tea's getting cold. Come on, Ace. We've got work to do."
The Seventh Doctor is perhaps the most egnimatic of the Doctors. Originally somewhat of a clownish, clumsy character, he soon became a darker persona, often manipulating his enemies and his companions for his own ends. Apart from this, he liked to play with spoons and enjoyed blues and jazz.
However, it was during McCoy's era that the show continued to decline ratings-wise and Doctor Who was canceled after over twenty-five years. Nevertheless, fans were adamant about bringing it back. A novel series continued the adventures of the Seventh Doctor, and the BBC explored options to bring the Doctor back, perhaps through an independent company. Eventually, they struck a deal with FOX to produce a television film, which *might* lead to a new series. The movie would be aired in Britain and America, and was a co-production.
For the film, McCoy returned to reprise his role as the Seventh Doctor, who is escorting the remains of his enemy, the Master, to Gallifrey, his home planet. However, the Master isn't quite dead and the TARDIS is forced to materialize in the middle of a San Francisco gang war. The Seventh Doctor is shot, and the Master posesses his Ambulance driver. Eventually at the hospital the Doctor regenerates into:
The Eigth Doctor (Paul Mcgann) 1996
"I love humans. Always seeing patterns in things that aren't there."
Very energetic and full of an enthusiasm about life, the Eigth Doctor only appeared once in the TV movie, although the adventures of the Eigth Doctor, with new companions and enemies, continues in novels and audio dramas(Where Mcgann reprises his role).
Some major spoilers for the new series:
Sometime during this Doctor's life, a massive time war erupted between the Daleks and Gallifrey. The Doctor, in order to bring an end to the war, presumabely destroyed both Gallifrey and the Dalek fleet, an incident which also caused him to regenerate into the Ninth Doctor
Eventually, new management took over at the BBC that was favorable towards the show, and decided that it was time to bring it back. The New Doctor Who series debuted to strong ratings and was mostly well-received, and has been renewed for at least two more years and for two holiday specials. The Second season and Holiday special are currently in production. The new series started off with:
The Ninth Doctor (Christopher Eccleston) 2005
"I saw the Fall of Troy, World War Five... I pushed boxes into Boston Harbor at the Boston Tea Party. Now I'm gonna die in a dungeon... In Cardiff!"
SPOILERS for the new series:
This Doctor, under his energetic nature and sense of humor, also harbored a deep sadness about the fate of his homeworld and a deep hatred of the Daleks. He cared deeply for his companion, Rose, and was somewhat distrustful of most male company she kept, including her boyfriend Mickey and alien expert Adam, although he eventually found an ally with companion Captain Jack. Eventually, in order to stop the Daleks, Rose absorbed the power of the time vortex into herself, which proved too powerful for her frail human body. The Doctor sucked the energy out of her and dispersed it, but in doing so damaged every cell in his body. He regenerated into
The Tenth Doctor (David Tennant) 2005-2010
"Let's RIP through that vortex!"
The Tenth Doctor is also manic and very often gleeful as he dashes about time and space. His first companion is Rose Tyler, before later being accompanied by Martha Jones and Donna Noble. To date he has confronted the Daleks three times, the Master once and also encountered Cybermen from a parallel universe (more below). He's also met Davros once more, seen the Fourth Doctor's companion Sarah-Jane Smith again and started lying about his age. He will appear in 3 (or 4) specials in 2009 before regenerating into:
The Eleventh Doctor (Matt Smith) 2010-???
The youngest actor to play the part yet. (cue fanboy screams)
Some of the Doctor's companions:
The Doctor's companions (Well, a few of them, at least)
Susan (Carole Ann Ford) 1963-1964
The Doctor's first companion, Susan was apparentally the Doctor's grandfather and a fellow exile/renegade, although her exact past and relationship with the Doctor has never really been explained. She left the Doctor when she fell in love with an Earth resistance fighter in "The Dalek Invasion Of Earth" and returned for the special "The Five Doctors".
Sarah Jane Smith (Elisabeth Sladen) 1974-1976
For many the definative companion, Sarah Jane Smith first appeared during Pertwee's last season and stayed with Tom Baker for two and a half years. A journalist, she enjoys adventuring with the Doctor and has a sense of humor. She's also a bit of a feminist, although this is toned down in her later stories. She returned in the special "K-9 and Company" as well as "The Five Doctors" and made a return appearence in "School Reunion" in 2006. She subsequently got her own spin-off 'The Sarah-Jane Adventures' which is aimed at kids, but brilliant anyway.
Leela (Louise Jameson) 1976-1977
A Savage human from the future, the Doctor brings her onboard the TARDIS where he acts as a semi-mentor to her on the ways of science instead of magic. She's a strong-willed warrior woman, sort of a Xena before Xena.
Romana (Mary Tamm, Lalla Ward) 1978-1981
A timelady from Gallifrey, she initially was sent to help the Doctor recover the Key To Time, after that quest was completed, she stayed on until the Doctor ended up in the parrarel universe known as E-space, where she stayed. Romana is about equal with the Doctor in terms of intelligence, although she initially lacks the adventuring experience. Later, when she regenerates into a new form, she becomes much more Doctor-like in her personality.
Brigidiar Alistar Gordan Lethbridge Stewart (Nicholas Courtney) various
Head of UNIT, the Brigidiar first appeared in the second Doctor era, and was featured prominently in the Third Doctor era. He made appearences in the Fourth, fifth, and Seventh Doctor eras but has been absent from the sixth, eigth, and ninth Doctor eras although the first two have been rectified by audios and novels. He didn't appear in the First Doctor's era (Although the actor did play a character) but he's met the first Doctor in the multi-Doctor stories.
The Brigidiar is perhaps the Doctor's most constant ally on Earth, and one of his good friends, although they do tend to get on each other's nerves-especially when the Brigidiar feels that force is the best solution when the Doctor would settle for a more peaceful way out.
Josephine "Jo" Grant (Katy Manning) 1971-1973.
The companion mostly closely identified with the Third Doctor, Jo wasn't the brighest companion, but still had a good heart and became good friends with the Third Doctor.
Tegan Jovanka (Janet Felding) 1981-1984
Tegan appears in all but one of the Fifth Doctor's stories. She's an air hostess from Australia who often complains about the Doctor's inability to control the TARDIS, and has suffered considerable trauma on her journeys, being posessed by the entity known as the Mara at least twice.
Perpigilliam "Peri" Brown (Nicola Bryant) 1984-1986
Initially a companion of the Fifth Doctor, it wasn't long after they met that he regenerated into the Sixth. Peri had a somewhat difficult relationship with the Sixth Doctor, but for all their arguing there was friendship between the two.
Dorothy "Ace" Mcshane (Sophie Aldred) 1987-1989
This troubled teen was taken from her home by a time storm and wound up on the planet Iceworld, where she first met the Doctor. She often talked in a lot of street slang and often did a lot of physical stuff, tackling the Doctor's enemies with a variety of weapons. She also became sort of a pawn in the Seventh Doctor's games at times, which put somewhat of a strain on their relationship.
Rose Tyler (Billie Piper) 2005-2006 (2008)
One of the Doctor's most recent companions, an intelligent but slightly typical teenager. She enjoys her travels with the Doctor but it sometimes puts a strain on her domestic life. She wound up being sucked into a parallel universe at the end of Season 2, but returned in Season 4 for the finale. She then wound up back in the parallel universe.
Captain Jack (John Barrowman) 2005 (intermittent appearances since)
A bisexual from the 50th century, Jack's sort of a man's man and a Ladies's man. He first meets the Doctor and Rose during an adventure in 1940's London. He travelled briefly with the Doctor at the end of S1 before being killed battling the Daleks. He was resurrected by Rose, but left behind. He then got his own spin-off, 'Torchwood', before appearing in the final three episodes of S3 in which it was revealed that he had a time-jump device which he used to jump to the late 19th century and that he apparently can't be killed because of Rose's ressurection of him. He returned to Torchwood, but appeared again in the 2-part finale of S4.
The Doctor's enemies:
The Daleks. The Mutated survivors of a nuclear war, the Daleks were encased in metallic war machines by their creator Davros. Each Dalek is encased in thick armor, carries one powerful weapon (Capable of scrambling one's insides) and a manipulater "plunger" sucker arm capable of crushing skulls or cracking a combination very quickly. Later generations of Dalek have force fields and are capable of flight to get over obstacles. The Daleks believe they are the superior beings in the universe and wish to either rule over or destroy all others.
Davros. Creator Of The Daleks, Davros is pretty much the model on which they are based as well. Initially believe killed by his creations, he eventually was revived and brought some of the Dalek race under his control, creating a civil war in the process. He reappeared in S4's finale where it was revealed he'd created a new race of Daleks from his own DNA.
Roger Delgado 1971-1973
Peter Pratt 1976
Geoffrey Beevers 1981
Anthony Ainley (Pictured) 1981-1989
Eric Roberts 1996
Sir Derek Jacobi 2007
John Simm 2007
The Doctor's arch-nemesis, a renegade time lord who is bent on universal domination. He's also a bit of a body-snatcher as well, since he's exhausted all his lives and seeks to extend his life in anyway possible.
The Master first appeared during the Third Doctor's era, appearing in a large number of stories. He later showed up in the Fourth Doctor's era as a walking corpse before gaining a final body towards the end of Tom Baker's run. In this new body he caused trouble for the fifth, sixth, and Seventh Doctors. He gained another new body in the TV movie and caused problems for the recently regenerated Eigth Doctor, before being sucked into a black hole...he was revived during the Time War by the Time-Lords, but fled to the end of time and disguised himself as a human and forgot who he was. He subesquently found out, stole the TARDIS, regenerated and travelled back to the early 21st century where he got elected PM of Britain and then took over the world for year. Which was undone in a huge reset button.
The Cybermen are from the Planet Mondas, which replaced most of their body parts with cybernetic replacements, a process which also eliminated their emotions. The Cybermen are bent on conquest, not only of Earth but also of humans themselves, who they wish to convert into Cybermen. Cybermen have the appearence largely of humanoid robots but still posess living matter in their bodies. They have a largely military structure, with most squads being led by "Cyberleaders" who are distinquished by their black headsets.
S2 introduced Cybermen from a parallel universe Earth created by a lunatic businessman who wanted to avoid dying. These Cybermen were defeated, but returned in the finale where they tried to invade 'our' universe, battled the Daleks and were subsequently sucked in to an interdimensional void. A group of them wound up in 1851 and appeared in the 2008 Christmas special.