Doctor Who: Three
The Third Doctor: Jon PertweeCasting Jon Pertwee as the Doctor was very nearly the opposite of casting Patrick Troughton -- Troughton did scary movies and played a hobo space clown, and Pertwee, known for things like A Funny Thing Happened On The Way To The Forum and innumerable Carry On films, ended up playing his gentleman-adventurer Doctor almost perfectly straight. Pertwee, comedic career notwithstanding, was an ex-Navy man and had a passion for things that went vroom, which came out in the Third Doctor's sheer glee at things like hovercrafts and open roadsters. Pertwee himself was an avid biker and rode around on fast motorbikes well into his 70s.
First Serial: Spearhead From Space
Last Serial: Planet of The Spiders
Costume: Edwardian gentleman, as interpreted by the 1970s. Tuxedo trousers with a flare at the cuff, velvet smoking jackets, great sweeping opera capes and ulster coats. Don't forget at least one pocket large enough to hold an assortment of gizmos, including the sonic screwdriver.
Companions: Liz Shaw, Brigadier Alastair Gordon Lethbridge-Stewart, Jo Grant, Captain Mike Yates, Sarah Jane Smith.
Regenerated by force and separated from both his friends and some important parts of his TARDIS, the Third Doctor discovered very quickly that one can get quite irritated with even one's favorite little planet, if one is sentenced to house arrest on it for long enough.
Fortunately, his old friend Lethbridge-Stewart, now a Brigadier, happened to be working with UNIT, which was handed all the strange meteorites and alien invasions by default. The Doctor was not keen on the idea of having any kind of official position or ID at first -- the poor man sent in to do his paperwork opted to believe him when he said his name was John Smith, just to get it over with -- but once they'd given him an assistant or three, he did eventually warm to the idea of protecting the Earth in a rather more linear fashion than he had been. The Third Doctor was rooted in once place long enough to have friends in addition to companions; several of the UNIT officers and enlisted became regulars, and the Brig is one of the few people to have met him repeatedly in many incarnations, and often out of order.
Though Liz Shaw was a bit too knowledgeable and headstrong in her own right to mesh well with the Doctor, and Jo Grant somehow turned into an enthusiastic ditz instead of a kung-fu action girl, the third lady to run into this Doctor has become a fan favorite: Sarah Jane Smith. Doctor Who had been sort of sneaking in the idea of competent women since Verity Lambert was at the helm, but Sarah Jane made her feminist points loudly and explicitly, and frequently directly to the Doctor's face. She was very much an Earth girl, so character-wise didn't duplicate too many of the Doctor's specialist scientific skills, but as an investigative reporter, she was also more than willing to do her own breaking and entering and hitting things and so forth and so on.
(Side note: You may notice that 'investigative reporter' is an oddly common trope in early feminist female characters. 'Spy' also pops up a lot, which is pretty much the same thing only with fewer editors. Making a woman clever and devious was acceptable a lot earlier than making a woman hard-punching and violent. She was essentially a femme fatale, a role with which the audience was already familiar, only working for the good guys.)
Lis Sladen, the actress who played Sarah Jane, stayed very much attached to the franchise until her untimely death in 2011 actually ended her spinoff series The Sarah Jane Adventures. Doing an episode of the new series was rather poignant for both her and David Tennant -- for her, because she was the first of the classic actors to be called back, and for him, because he remembered her so vividly as Sarah Jane from when he was a child.
Another consequence of the Doctor being stuck on one planet is that he developed an actual archnemesis -- an individual, personalized one, with whom we later find out he went to school. The Master, as portrayed by Roger Delgado, is one of the most deliciously evil characters in space opera, and after Delgado was killed in an accident in 1973, was never quite the same. (The later Master played by Tony Ainley inherited his black gloves and facial hair from Delgado.) Out of character, Delgado was a lovely man and quite adored by the rest of the cast. You can see him in the background in some of the infamous rock quarries, "kidnapping" Jo very carefully down the rocky slopes, as Katy Manning was blind as a bat without her glasses, and he didn't want her to slip.
The Third Doctor's run was the first to include color serials; up until the 1970s, the show was still taped in black and white. This has posed a slight problem with some proposals to repeat the early episodes in it, as television stations generally don't want B&W episodes mixed in with color -- one of the reasons you don't often see the first season of things like The Man From Uncle either. Of particular concern are the episodes of full color serials where only black and white overseas sale copies survive, the color tapes having been junked. Fortunately, most of them survive, including The Three Doctors, the first of several such specials that involve calling back the Doctor's previous regenerations to team up and save the universe, usually despite themselves.
The Third Doctor, combining misdirection and Venusian Aikido.
Establishing the long-standing tradition that the Doctor irritates himself to no end.
The Third Doctor's final goodbye to Sarah Jane.