On Earth, the 20th century, recorded as 2.0K by the 43rd century dating system, (AUDIO: The Torchwood Archive) was one of the most turbulent, violent, yet progressive centuries in the planet's history. In addition to major conflicts including the First (TV: To the Last Man) and Second World Wars, (PROSE: Players) humankind began to explore beyond their homeworld for the first time, and also began to make huge leaps in technological advancement. (TV: Day of the Moon) It was also during the 20th century that many world governments learned of the existence of extraterrestrial lifeforms. (AUDIO: Storm Warning, TV: Spearhead from Space) According to many accounts the general public also learned of alien life, (COMIC: Invaders from Gantac!, The Mark of Mandragora, PROSE: The Dying Days, Soldiers from Zolta) although other accounts indicate they remained in the dark about this until the beginning of the 21st century. (TV: Aliens of London)
Numerous alien defence organisations came to power in this century, including the Torchwood Institute, (TV: Fragments) the Forge, (AUDIO: Project: Twilight) LONGBOW, (PROSE: Just War) Counter-Measures, (TV: Remebrance of the Daleks) C19, (PROSE: Who Killed Kennedy) HAVOC, (PROSE: The Showstoppers) UNIT, (TV: The Web of Fear, The Invasion) MIAOW, (PROSE: The Dreadful Flap) P.R.O.B.E., (HOMEVID: The Zero Imperative) and UNISYC. (PROSE: Interference)
The Modulars believed that Earth in this era was a complete temporal mess due to the high amounts of aliens and time travellers. The planet was constantly being destroyed, replaced, removed from history, but the timelines somehow always stablisied. (PROSE: Elementary, My Dear Sheila)
The climate and air quality on Earth began to deteriorate during this century. (AUDIO: The Dread of Night)
The Doctor had a great affinity for this century, though his first incarnation initially claimed to tolerate but not enjoy it. (TV: "An Unearthly Child") However, his granddaughter Susan Foreman believed that he had a fondness for the 20th century as he often visited it on their travels together. (AUDIO: The Alchemists) He lived during this time period for several years as an exile from Gallifrey (during his third incarnation), (TV: Spearhead from Space Doctor Who and the Silurians, et al.) and again for a number of months in 1913 and 1969 in his tenth incarnation (to name just two examples). (TV: Human Nature / The Family of Blood, Blink) By the Doctor's fourth incarnation, his affinity for 20th century Earth was so well known that anyone looking for him would first think to search there. (AUDIO: The English Way of Death) Many of the Doctor's Earth companions came from this century, as he visited it often. (TV: An Unearthly Child, The Massacre of St Bartholomew's Eve, The War Machines, Spearhead from Space, Terror of the Autons, The Time Warrior, et al.)
At some point in the 20th century, the Fisher King invaded the planet Tivoli and was there for ten years. The native Tivolians were liberated by the Arcateenians, but later enslaved by the very same race. (TV: Before the Flood)
As with most centuries of the first two millennia, the 20th century was home to Jack Harkness, Amy Pond and an Auton duplicate of Rory Williams. A version of Jack from around the time of the deaths of Toshiko Sato and Owen Harper existed in this century, having been buried alive in the 1st century by his brother, Gray. He perpetually died and resurrected an unknown number of times in an earthen tomb underneath Cardiff. (TV: Exit Wounds) Meanwhile, a near-dead Amy Pond was kept alive inside the Pandorica, beginning in the 2nd century. An Auton version of Rory kept vigil near her the entire time. They both awaited a moment in the mid-1990s when a young Amelia Pond would touch the outside of the Pandorica and restore Amy to full health. (TV: The Big Bang)
It was substantially unclear whether the events of the subsequent Big Bang Two erased Amy and Rory's presence in the 20th century. This ambiguity was caused, in part, because the non-Auton, married Rory Williams claimed to have remembered being "made of plastic" at his wedding reception, suggesting that, at least inasmuch as he was concerned, he and Amy were present in the 20th century. (TV: The Big Bang) Amy seemed to also remember those events, and displayed a fondness for the Auton Rory both during her honeymoon (TV: A Christmas Carol) and during a kind of lullaby to her newborn child Melody Pond. (TV: A Good Man Goes to War)
It was also among the centuries endured by Ashildr, (TV: The Woman Who Lived) a 9th century[1] Viking girl who was rendered effectively immortal when she was brought back to life by the Twelfth Doctor through a self-repairing Mire repair kit. (TV: The Girl Who Died)
By the year 3012, the 20th century was considered ancient history. (PROSE: Festival of Death)