Time Lords: Regeneration, Gallifrey and Two Hearts Explained

The Doctor's people were a mystery for as long as the series could keep it. From November 23, 1963 the character was an old man in a police box who had, he said, been cut off from his own people; "The War Games" (1969) brought those people on screen at last, as the Time Lords, robed judges who tried the Doctor for interference, exiled him to Earth and forced his second regeneration. Gallifrey was named in "The Time Warrior" (1974) and visited in "The Deadly Assassin" (1976), which showed a Capitol under a dome on a world of red grass and silver-leaved trees, a Panopticon, a President, the founders Rassilon and Omega and a rigid, ceremonial society that had sworn off intervening in the universe after building the machines that let it. The Doctor stole one and left, which is the whole premise.

The 2005 revival destroyed the planet before showing it. The Time War against the Daleks, fought across time itself, ended, the Doctor believed, when he burned both sides; "The Day of the Doctor" (2013) revised that to a rescue, with Gallifrey frozen in a pocket universe, and "Hell Bent" (2015) brought it back. "The Timeless Children" (2020) then had the Master destroy it again, along with the species' account of itself: the Time Lords had not evolved regeneration but had taken it from a foundling child from another universe, and the child was the Doctor.

Regeneration was invented to keep the show on the air. William Hartnell, the first Doctor, was too ill to continue in 1966, and the producers wrote a scene in which the Doctor collapses and rises as a different man, Patrick Troughton; the process was called renewal, then regeneration from 1974, and it has since let sixteen actors play one character across six decades. The rule of "The Deadly Assassin" (1976) that a Time Lord may regenerate twelve times became a plot problem by 2013, when the count ran out, and a gift of a new cycle from the Time Lords solved it; the 2020 revelation that regeneration was the Doctor's own biology, extracted by the Time Lords' founders and spliced into their species, removed the limit and the species' claim to the process at once.

The rest of the physiology accumulated as scripts needed it. Two hearts arrived in 1970 when a hospital doctor listened to the Doctor's chest; a respiratory bypass, a body temperature of 15 degrees and the ability to survive vacuum for minutes followed; a Time Lord can sense the turning of the universe, see the timelines of a person at a glance and, after regenerating, spend hours in a fever with the wrong personality. Each new body is a different person with the same memories, which the series has used to explain everything from an actor's accent to a change of sex, and which it has also, in its better episodes, treated as a kind of death.

The TARDIS is the species' great machine and the Doctor's stolen one. Time And Relative Dimension In Space, the name coined by the Doctor's granddaughter in the first episode, is a ship grown rather than built, larger inside than out because the interior exists in another dimension, capable of any point in time and space and of disguising itself as anything, except that the Doctor's has a broken chameleon circuit and has been a London police box since 1963. It is alive, has a mind, chose the Doctor as much as the reverse, and in "The Doctor's Wife" (2011) spoke through a human body for an episode. The Time Lords' other machines, the Eye of Harmony, the Matrix and the Hand of Omega, are the reasons the species could fight a war across history.

The Time War is the revival's founding trauma. Fought between the Time Lords and the Daleks, it was locked away from the rest of time and ended, the Doctor believed for eight series, when he used a weapon called the Moment to burn both worlds; the guilt gave the ninth, tenth and eleventh Doctors their character, and the fiftieth anniversary undid the deed without undoing the guilt. The Timeless Child rewrote the species' place in that history: the Time Lords had built their civilization on a stolen child's biology, hidden it, and erased the child's memory, and the Doctor, learning this, has spent the seasons since deciding not to look.

The Doctor spent the 1970s working for the government, and the government was real. UNIT, the United Nations Intelligence Taskforce, introduced in "The Invasion" (1968), was a military body tasked with investigating the alien and the unexplained, and the exiled third Doctor served as its scientific adviser, driving to landed spaceships in a vintage car while the Brigadier called in the artillery; the arrangement was the series' version of Project Blue Book, the U.S. Air Force's investigation of 1952 to 1969, whose civilian scientific consultant, J. Allen Hynek, was an astronomer who went from explaining sightings away to founding a research centre. Hynek never got a police box, but the role, the scientist attached to the soldiers who keep the files, is his.

The Time Lords' machines are ufology's wish list. A ship larger inside than out that vanishes and reappears is the report of every witness who watched a light go from a standstill to gone; the anti-gravity claims made for such craft and the warp drive physics that might one day approach them are covered in their own articles, and neither includes time travel, which physics allows in principle for the future and forbids, so far, for the past. The Doctor's answer to the question of how the TARDIS works is that it is a Time Lord thing, which is also, in the end, the answer of the witnesses.