Hartnell's portrayal of the character was initially a stubborn and abrasive old man who was distrustful of humans, but ultimately mellowed out into a more compassionate, grandfatherly figure who adored his travels with his companions.

The First Doctor's original companions were his granddaughter Susan Foreman (Carole Ann Ford) and her schoolteachers Ian Chesterton (William Russell) and Barbara Wright (Jacqueline Hill).

In later episodes, he travelled alongside 25th-century orphan Vicki (Maureen O'Brien), space pilot Steven Taylor (Peter Purves), Trojan handmaiden Katarina (Adrienne Hill), and sixties flower child Dodo Chaplet (Jackie Lane).

His final on-screen companions were the working class sailor Ben Jackson (Michael Craze) and the sophisticated socialite Polly (Anneke Wills).

He has a ship that travels through time and space, the TARDIS, which is currently disguised as a police box (Susan notes that it used to be able to change to blend in with its surroundings), and is bigger on the inside.

The series' first episode opens with a pair of schoolteachers in contemporary (1963) London, Ian Chesterton and Barbara Wright, investigating the mystery of Susan, a student who seems confused and even frustrated at how what she is learning in history and especially mathematics seems to be wrong.

The TARDIS crew also observed many historical events such as the Reign of Terror in revolutionary France, meeting Marco Polo in China and The Aztecs in Mexico.

The Doctor and Steven were next briefly joined by Trojan slave Katarina and a security agent from 4000 AD Sara Kingdom, but both were killed during the events of The Daleks' Master Plan, where the Daleks plotted to invade Earth's Solar System in 4000 AD, before being destroyed when the Doctor activated their weapon the Time Destructor.

After narrowly missing the Massacre of St Bartholomew's Eve, the Doctor and Steven took on board a young girl named Dodo Chaplet.

An early writers' guide by script editor David Whitaker describes "Doctor Who" as "frail-looking but wiry and tough as an old turkey".

Eventually, the Doctor began to enjoy his travels through time and space, taking people along for the ride and was always reluctant and sad to see them go, even when he knew it was for their own good.

The Doctor's personality mellowed around the time of the serial Marco Polo, and he evolved into the more familiar compassionate grandfatherly figure that children loved, and was protective of the young women he took on as companions; they reminded him of his granddaughter, Susan.

[4] Sometimes this was a deliberate acting choice: William Russell recalls that it was Hartnell's idea for the Doctor to get Ian Chesterton's surname wrong, calling him "Chesserman", "Chatterton" and even "Charterhouse".

[7] In "Twice Upon a Time", writer Steven Moffat has the Doctor express to a variation of his future companion Bill Potts that he had left Gallifrey, among other reasons, to investigate why good prevails in a universe where evil would seem to have so many advantages.

[9] Scripts filled with far-out concepts compensated for the relatively low budget and unsophisticated special effects, laying the foundation for decades of stories to come.

Despite the regeneration television audiences would see the First Doctor on screen for several more occasions (not counting flashbacks or charity specials like Dimensions in Time).

Due to failing health, however, Hartnell could not participate in any of the regular filming, so his scenes were shot separately at Ealing Studios (not his garden or garage at home, as long suggested by fan legend); in the context of the episode, this is explained as the First Doctor being trapped in a time eddy when he was being sent to assist his future selves, with the result that he can only communicate with his future selves for brief moments over the TARDIS monitor while sitting in a capsule.

[13] Bradley had previously portrayed William Hartnell in An Adventure in Space and Time, a 2013 docudrama depicting the creation of Doctor Who.

He also makes a brief appearance (again using a double) during Missy's exposition of the Twelfth Doctor's battle with android assassins ("The Witch's Familiar").

In "The Big Bang", the Eleventh Doctor briefly mentions the First while bidding his farewell to a sleeping Amy Pond, referring to him as "the daft old man who stole a magic box and ran away".

In "The Devil's Chord" the Fifteenth Doctor visits 1963 and points out the Totters Lane junkyard off in the distance from atop EMI Recording Studios, saying he lived there with his granddaughter Susan.