|Let's find out.|
After the premature death of Robert Delgado, the Master's original actor, the planned story arc with the Master that would end in his death was aborted, and Jon Pertwee decided it was time to leave the series. The original idea was to cast an older Doctor, more like the 1st, but the people in charge saw Tom Baker's performance as the villain in The Golden Voyage of Sinbad and decided to go with him. Tom Baker was quite insistent on how he would play the Doctor. He would play him "like an alien." Boy, did he ever.
After 3 died of radiation poisoning, he changed forms once again. Once he recovered and defeated a giant, renegade robot, he and Sarah Jane Smith ran off in the TARDIS, leaving UNIT one science advisor short.
This Doctor was shown to be able to actually perceive changes to the time stream, along with other Time Lords. He was also shown to be able to remain conscious after instantly lethal injuries in the moments before regeneration.
Taken back home (well, to Aberdeen) after the Doctor began to fear for her safety.
Someone I didn't mention last time, Harry was a Royal Navy surgeon who worked for UNIT during the 3rd Doctor's tenure as their science advisor. Followed the 4th Doctor into the TARDIS after the giant robot incident. Left the TARDIS team after the defeat of the Zygons, taking a train back home. The guy was one stuffy Brit; brash and old fashioned, but brave and quite good at cricket. A shame; if he'd hung out with the 5th Doctor, he might have lasted longer.
- A robot dog companion built by Professor Marius, given to the Doctor as a gift. It was very protective of Leela, and stayed with her on Gallifrey.
- A second one was built, and protected Romana.
- A third was given to Sarah Jane Smith in the original Sarah Jane spin-off, K-9 & Company, later made canon by the episode "School Reunion."
- A fourth was given to her after the Mark III died in that same episode.
(Behind the scenes, Tom Baker would constantly kick the oft-malfunctioning prop and drop F-Bombs on it. The most hated companion ever behind the scenes (save Kamelion), but the one that was most beloved by the audience.)
Leela (Full married name: Leelandredloomsagwinaechegesima)
A space cave-woman. A member of a tribe descended from crashed spaceship survivors. She's not dumb, because she can understand sophisticated concepts, like why the TARDIS is bigger on the inside, she's just uneducated, which comes from living in a place with no education system that was at the mercy of an insane computer. She had a knife, and wasn't afraid to use it. She wasn't afraid to use poisonous thorns, either, but the Doctor put a stop to that. Chose to live on Gallifrey with K-9 and her Time Lord boyfriend, Andred.
Romanadvoratrelundar, a Time Lady fresh out of the academy. Smart and not afraid to put he Doctor in his place, she was assigned to the Doctor by the White Guardian to find the Key to Time. (More on that later.) Smart and able, she was a more "by-the-book" Time Lady, and would often butt heads with the Doctor on the operation of the TARDIS. Naive, but still a bit stubborn. She wanted to be addressed by her unwieldy first name, so the Doctor threatened to call her "Fred," a name she took a liking to. The actress (Mary Tamm) was told that she wouldn't be a damsel in distress, but eventually wound up in that role. She resigned and was replaced with a new actress, Lalla Ward.
Let's not get into the reasons why Romana regenerated in the show (if there is a reason, and this wasn't just out of the blue to replace the actress), but she regenerated into the form of someone that she and the Doctor had actually met: Princess Astra. Her personality was similar to her last incarnation, but she was less naive and had more respect for the Doctor.
Met the Doctor for one adventure, then started adventuring with him after they met again. An alien princess, she was more trusting and more mature than other companions. Her father's body was permanently taken over by the Master, yet she never tried to get revenge. She didn't dwell on her sadness, channeling it instead into more positive expressions of determination.
Aka "the Original Wesley Crusher." Adric was a young genius who won a badge for mathematical excellence. Nerd.
An Australian airline stewardess who wandered into the TARDIS in an attempt to call the police after the Master used his Tissue Compression Eliminator to kill her Aunt Vanessa. Panicky and often angry, she'll tell you exactly what's on her mind. She learned much from Nyssa about logic and determination. The Doctor keeps trying to get her back home, but keeps landing the TARDIS somewhere else.
A note: Just because an enemy appeared in an earlier Doctor's run doesn't mean it was notable. For example, 10 had a Weeping Angels episode, but I'm going to cover them only when we get to 11, because that was when they became a notable enemy and not just a one-off monster of the week.
The Cybermen, unwilling to be kept down for long, attempted to invade over, and over, and over, in greater numbers than ever before. Luckily, the Doctor and UNIT kept them at bay with weapons that fired gold dust (it clogs up their respirators and they suffocate).
The Doctor would notably be thrust back in time on a mission on the planet Skaro, where the Kaleds and the Thals were both engaged in a horrible war, producing mutants and aberrations. The Doctor's mission was to prevent the fascistic scientist Davros from converting Kaled mutants into Daleks. Ultimately, the Doctor chose to not commit genocide to carry out this goal, and merely set back the creation of the Daleks as opposed to wiping them out.
Davros was ruthless, racist, power hungry, and loud.
All these traits and more were shown by his most fearsome creations: the Daleks.
The Master returned in a horribly decayed body, having tried to extend his life after using up all of his regenerations.
He found a way to inhabit the body of Tremas, the ruler of Traken and Nyssa's father. From there, he resumed his schemes to takeover the universe, but instead committed the greatest act of wanton destruction until the stars started going out...
The Sontarans are a warrior race, like Klingons, Na'vi, Kazon, and more. However, the Sontarans are defined as being the most disciplined military in the universe. Their only weakness is getting hit in the back of their neck (their belly button, essentially, since all Sontarans are clones) which is why they can never turn their back on an enemy.
Locked in perpetual war with the Rutans, Sontarans often try to gain any advantage possible, from genetic experimentation, to time travel, which puts them at odds with the Doctor.
The Black Guardian
The personification of darkness, in opposition to the White Guardian. Attempted to steal the Key to Time after the Doctor had assembled it.
It seems that dying of radiation always makes the next Doctor a little loopy, as we'll see when we get to 11. 3 was stuck on Earth for most of his life, so the first thing 4 did was leave (after he defeated a giant robot).
As opposed to 3's rigid, uppercrusty personality, 4 was the most bohemian Doctor out of all of them. Even more than the Second Doctor. This Doctor was weird, but only by human standards. For example, he often smiled at inappropriate times (by human standards), and he would often make light of a bad situation by changing the subject. His behavior was a mish-mash of alien customs. Tom Baker didn't go halfway with anything, when it came to playing this alien like... well, like an alien.
"Would you like a jelly baby?"
Notable Character History
This was when Douglas Adams (yes, that one) took over the show. 4's run was marked by funny, but smart sci-fi adventures. This was the point where the show got... Bigger. There are very few "unimportant stories," compared to earlier, no-consequence-monster-of-the-week series.
After Sarah Jane and 4 traveled together for a while, her body was taken over by an evil alien. She got better, but the Doctor finally decided that enough was enough. He'd led enough people to their deaths. He took her back home (well, Aberdeen, so close enough), and went off to Gallifrey, where he avoided being framed for the murder of the Time Lord President by running for president. It makes sense in context. The Master was behind it, deformed and dying after running out of regenerations. After the Doctor spent a small amount of time without a companion (because Tom Baker was convinced that he could carry the show solo by talking to the audience... which he did.), he defeated an evil computer possessing a flawed copy of his own mind (don't ask) and gained a new companion: Leela, who he had several adventures with.
After parting ways with Leela, the Doctor was then charged by the White Guardian (an omnipotent being on the side of order) to find the six pieces of the Key to Time, an item of infinite power split comprised of seemingly random artifacts strewn across all of space and time. He was given a new companion, Romana, and he succeeded in his quest, but destroyed the Key when he discovered that the White Guardian who showed up to take the Key was actually the Black Guardian.
The Doctor and a newly regenerated Romana went off into time and space, and had many, many, many more adventures, until Romana decided to help the people of E-Space (Exo-space; a pocket universe).
The Doctor then kept foiling the Master's evil schemes, with Nyssa, Adric, and Tegan joining him on the way. As the Doctor grew older, he started lamenting his long life. His thoughts kept dwelling on entropy, and the vast scope of eternity. He made his way to Logopolis, in order to fiddle with the TARDIS exterior. Logopolis was a city full of mathematicians who specialized in block-transfer computations. Basically, a team of hundreds of people would all do different parts of the same immensely complex math problem in a very specific arrangement. The resulting math was so powerful that it had to be done by hand because the very equations themselves had the power to change reality.
The Master attempted to take over the universe using Logopolis, but accidentally destroyed the city. With the end of the universe approaching, the Master and the Doctor had to team up to save reality. It was demonstrated to the Doctor that the universe should have already ended. A long, long time ago. As it turns out, the people of Logopolis were using their equations to halt the increase of entropy. The Master betrays the TARDIS team, however, and manages to wipe out a large portion of existence.
The Doctor manages to follow the Master to Earth, where the Master's using the gigantic dish of the Pharos Project to further his goals. He's essentially holding the universe for ransom, with the ability to destroy it at a moment's notice.
The Doctor manages to climb out onto the dish and defeat the Master by disconnecting the power cable, but the Master uses his controls to tilt the dish. The Doctor fell all the way to the ground as the Master escaped in his own TARDIS. Lying on the ground, surrounded by his companions, he gave his final words.
"This is the end. But the moment has been prepared for."
After merging with a time-displaced-quantum-possible-version of himself (who had appeared at this moment, and sent back in time to help the TARDIS team), he regenerated.
...I got nothing.
Um... He appeared in The Simpsons?
He teamed up with Captain Kirk to beat up Cybermen? And I'm not just talking about that drawing I made.
However, that's not official canon, so I shall elaborate no more.
Next time... An older Doctor, but also a younger one. See you then!