The Doctor has been played by other actors in other media: theatre plays, parodies, etc. These are not officially part of the "Doctor Who" story.
The movies "Dr. Who & the Daleks" and "Daleks' Invasion Earth 2150 A.D." star Peter Cushing as a human scientist named "Doctor Who". Shortly after it was announced that the series would return from its nineties hiatus, Richard E. Grant voiced The Doctor in an animated serial. The BBC issued press releases identifying Grant as the "ninth Doctor" but this was later disregarded by the revived BBC television series.
Sometimes different incarnations of the Doctor meet each other. Usually episodes with several Doctors are made when the program celebrates an anniversary. In 1973, when the show was 10 years old, the episode "The Three Doctors" was shown. It had the first three Doctors.
In 1983, there was the 20th anniversary special. "The Five Doctors". Patrick Troughton, John Pertwee, Peter Davison and Richard Hundall (replacing William Hartnell) played The Doctor. Tom Baker refused to play in it, so they used the old footage of the incomplete "Shada" episode to show him.
In 1985, there was the episode "The Two Doctors", with the Second and the Sixth Doctors.
The fifth Doctor met the tenth incarnation in mini-episode named "Time Crash". It was aired in 2007.
The Tenth, Eleventh, and War Doctors met each other in the 50th anniversary special on November 23, 2013. That was the first time the War Doctor was seen. That special had short appearances by all the other Doctors with the use of old, recycled footage. It also had a mysterious version of the Doctor from the far off future played by Tom Baker. The Eighth Doctor also made a return in 2013 for a 50th anniversary special mini-episode.
On August 4, 2013, Peter Capaldi was revealed as the actor who would play the Twelfth Doctor.
On July 16, 2017, it was revealed that Jodie Whittaker would replace Capaldi as the Doctor. She is the first female Doctor.
The TARDIS (Time And Relative Dimension In Space) is the Doctor's time machine. It can travel to any point in time and space, but the Doctor can never fully control it. The Doctor explains that TARDISes are not built, but they are grown like plants. The TARDIS is alive and has a mind of its own. In the episode "The Doctor's Wife" when the Doctor is able to communicate directly with its mind, the TARDIS explains that it did not always take him where he wanted to go but that "I always took you where you needed to go."
The TARDIS is bigger on the inside than the outside, which is commented on by almost everyone who enters. On the inside, the TARDIS looks like a giant spaceship, but on the outside it looks like a police telephone box. These telephone boxes were still a common sight in many British cities when the series first began in 1963, but after the walkie-talkie was invented they were no longer needed. They were used by the police as workstations - and people could also use them to call the police.
The Doctor stole the TARDIS from a Time Lord museum when he began his travels. It was described as being a rather old "Type 40" model, whereas the best version is the "Type 70" used by the upper class Time Lords. The TARDIS has a device called the "chameleon circuit" which is supposed to change its shape on the outside to allow it to disguise it. In the very first story, this device stopped working and left the TARDIS stuck in the shape of a police box.
The farthest the Doctor has ever travelled in the TARDIS is to the Big Bang (the beginning of the Universe) and 100 trillion years into the future (the end of the Universe) (in the episode "Utopia"). The inside of the TARDIS is much bigger than its outside (it is "dimensionally transcendental"). It has a lot of different rooms. The most important of which is the "console room" where the Doctor pilots the TARDIS.
The Doctor usually takes other people with them, who are usually called "companions" or "assistants". The Doctor and their companions travel through space and time, have a lot of adventures, and often save many people. The character of the companion was there so that the people watching the series could identify and feel close to a character. The companions were often present-time humans, so people could feel close to them, and they knew as much as the viewers. Because of this, the Doctor could explain things to their companions, and at the same time to the viewers. Almost all the Doctor's companions have been human, or human-looking aliens. Two companions were robots.
Overview: The BBC's link to a visual summary of Dr Who's opponents:
The Master is a renegade Time Lord. He is the Doctor's nemesis. He was conceived as "Professor Moriarty to the Doctor's Sherlock Holmes". The character first appeared in 1971, played by Roger Delgado. He continued in the role until his death in 1973. The Master was briefly played by Peter Pratt and Geoffrey Beevers until Anthony Ainley took over. He continued to play the character until Doctor Who's "hiatus" in 1989. The Master returned in the 1996 television movie of "Doctor Who", played by Gordon Tipple in the ultimately unused pre-credits voiceover, then Eric Roberts. The Master was in the three-part finale of the 2007 series, portrayed by Derek Jacobi. He regenerated into John Simm at the end of the episode "Utopia". Simm next appeared in the last (and two-part) episode of the Tenth Doctor in 2009/2010. He then regenerated off-screen into a "Time Lady" (Michelle Gomez), who returned for the Twelfth Doctor's first series in 2014.
The Daleks are one of the most powerful races in the Doctor Who history. They are the Doctor's worst enemies. They cry "EX-TER-MIN-ATE!" when they are about to kill someone. They look like small tanks or very large pepper pots. They first appeared in the first season episode "The Daleks". A more recent episode was "" (2019). They are not robots, but horrible aliens kept in a metal casing. They were mutated from Kaleds by their creator Davros during a war on their planet, Skaro. They can not be easily hurt by guns because their casing reflects or destroys the bullets.
Cybermen are another enemy of the Doctor. They are one of their worst enemies. In the original series, they come from the planet Mondas. In the new series, some of them come from a parallel universe. Cybermen travel across the universe taking people and turning them into machines. They stomp around saying "DELETE!". They kill by electrocution. They are partially vulnerable to the metal gold, and can be killed by gold bullets. They change their look over time, but stay recognizable by "handlebars" on their heads, and tear drops in their eyes.
The Doctor has faced them many times. The first was in "The Tenth Planet" (William Hartnell's last episode). A more recent episode was "World Enough and Time/The Doctor Falls" (the last two episodes of Peter Capaldi's last season).
The Sontarans are a group of aliens, bred in clone batches, that believe in war over anything else. They must face their enemy in combat because of their weak spot on the back of their neck, the probic vent. They have been said to look like baked potatoes.
The Doctor first met them in "The Time Warrior" (Jon Pertwee). They have been on the show and its spinoffs repeatedly.
The Sea Devils and the Silurians lived in the time of the dinosaurs until the catastrophe of the Moon's approach drove them into hibernation. They slept longer than planned and emerged late in the 20th century. The human scientist who discovered the Silurians estimated their era incorrectly. A later "Doctor Who" story said that they should have been classified as the Eocenes. A second species of hibernating Earth reptiles lived primarily underwater. They were labeled "sea devils" by a frightened, superstitious construction worker who encountered them.