The Second Doctor is the second incarnation of the protagonist

Doctor (Doctor Who)

The Doctor is the central character in the long-running BBC television science-fiction series Doctor Who, and has also featured in two cinema feature films, a vast range of spin-off novels, audio dramas and comic strips connected to the series....

 of the long-running BBC

BBC

The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...

 television science-fiction

Science fiction on television

Science fiction first appeared on a television program during the Golden Age of Science Fiction. Special effects and other production techniques allow creators to present a living visual image of an imaginary world not limited by the constraints of reality; this makes television an excellent medium...

 series Doctor Who

Doctor Who

Doctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a time-travelling humanoid alien known as the Doctor who explores the universe in a sentient time machine called the TARDIS that flies through time and space, whose exterior...

. He was portrayed by character actor

Character actor

A character actor is one who predominantly plays unusual or eccentric characters. The Oxford English Dictionary defines a character actor as "an actor who specializes in character parts", defining character part in turn as "an acting role displaying pronounced or unusual characteristics or...

 Patrick Troughton

Patrick Troughton

Patrick George Troughton was an English actor most widely known for his roles in fantasy, science fiction and horror films, particularly in his role as the second incarnation of the Doctor in the long-running British science-fiction television series Doctor Who, which he played from 1966 to 1969,...

.

Within the series' narrative, the Doctor is a centuries-old Time Lord

Time Lord

The Time Lords are an ancient extraterrestrial race and civilization of humanoids in the British science fiction television series Doctor Who, of which the series' eponymous protagonist, the Doctor, is a member...

 alien from the planet Gallifrey

Gallifrey

Gallifrey is a fictional planet in the long-running British science fiction television series Doctor Who and is the homeworld of the Doctor and the Time Lords...

 who travels in time

Time travel in fiction

Time travel is a common theme in science fiction and is depicted in a variety of media. It simply means either going forward in time or backward, to experience the future, or the past.-Literature:...

 and space in his TARDIS

TARDIS

The TARDISGenerally, TARDIS is written in all upper case letters—this convention was popularised by the Target novelisations of the 1970s...

, frequently with companions

Companion (Doctor Who)

In the long-running BBC television science fiction programme Doctor Who and related works, the term "companion" refers to a character who travels with, and shares the adventures of the Doctor. In most Doctor Who stories, the primary companion acts as both deuteragonist and audience surrogate...

. When the Doctor is critically injured, he can regenerate

Regeneration (Doctor Who)

Regeneration, in the British science fiction television series Doctor Who, is a biological ability exhibited by Time Lords, a race of fictional humanoids originating on the planet Gallifrey. This process allows a Time Lord who is old or mortally wounded to undergo a transformation into a new...

 his body; in doing so, his physical appearance and personality change.

The transformation into the Second Doctor (originally referred to as a "renewal"), a figure who was the same essential character as the first but with a very different persona, was a turning point in the evolution of the series, which eventually became a critical element of the series' longevity.
The First Doctor

First Doctor

The First Doctor is the initial incarnation of the protagonist of the long-running BBC television science-fiction series Doctor Who. He was portrayed by the actor William Hartnell from 1963 to 1966. Hartnell reprised the role in the tenth anniversary story The Three Doctors in 1973 - albeit in a...

 grew progressively weaker while battling the Cybermen

Cyberman

The Cybermen are a fictional race of cyborgs who are amongst the most persistent enemies of the Doctor in the British science fiction television series, Doctor Who. Cybermen were originally a wholly organic species of humanoids originating on Earth's twin planet Mondas that began to implant more...

 during the events of The Tenth Planet

The Tenth Planet

The Tenth Planet is a serial in the British science fiction television series Doctor Who, which was first broadcast in four weekly parts from 8 October to 29 October 1966. It was William Hartnell's last regular appearance as the First Doctor, and the first story to feature the Cybermen...

and eventually collapsed, seemingly from old age.