Patrick Troughton (right) was the first replacement for William Hartnell's original Doctor (left), appearing in the episode "The Power of the Daleks" -- in which the Time Lord and his companions face the murderous robots for the first time.
In "Spearhead from Space," Jon Pertwee (far right) was the face on the other side of the Doctor's new regeneration following the exit of Troughton. He's immediately thrown into conflict when he must explain himself to the police while dealing with Autons -- plastic beings controlled by the Nestene Consciousness.
"Robots" saw Pertwee (left) be succeeded by Tom Baker (next slide), who had to take on giant robots for his first challenge as the Doctor.
After Baker (right) left the show, Peter Davison (left) took up the mantle and found his first episode, "Castrovala," match him up against the Doctor's biggest foe, The Master. The Master took advantage of the Doctor's new regeneration to trap him in a fictional world.
Colin Baker's (right) first adventure as the Doctor, "The Twin Dilema," saw him confused -- and violent -- after his regeneration all while trying to solve the kidnapping of twins by a slug-like creature named Mestor.
Sylvester McCoy (right) would take over for Colin Baker as the seventh Doctor, and the last before the series would be suspended by the BBC. In "The Time of Rani" he had to deal with the evil Rani, who sought to control time travel with the help of the world's greatest minds.
In 1996, BBC and Fox did a made-for-TV-movie that saw Paul McGann (right) take over for McCoy (left). It would be McGann's only appearance as the Doctor, but he faced the Master once again.
2005 saw a revamped "Doctor Who" come back to BBC with Christopher Eccleston now in the TARDIS, taking over for McGann (left). The first episode, "Rose," saw him deal with the Nestene Conciousness, now in the control of mannequins.
While the regeneration that saw Eccleston's Doctor (left) turn into David Tennant's (right) had a seven-minute film introducing him, it was an invasion from a group of aliens known as Sycorax - on Christmas no less -- that would be the first official outing for Tennant.
Tennant's popular run as the doctor ended and saw Matt Smith (right) take his place in "The Eleventh Hour." Smith would have to contend with Prisoner Zero, a shape-shifting alien wanted by the galactic police.