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"Captain Jack Harkness" was the alias adopted by Time Agent and con man Javic Piotr Thane (AUDIO: Month 25) from the 51st century who became an associate and occasional companion of the Doctor.

After he was killed by a Dalek, he was revived by Rose Tyler, who at the time was transformed into a nearly omnipotent being. Unable to control her powers, she accidentally turned him into an immortal being. Because of this, the Ninth Doctor saw his companion as a fixed point — a temporal paradox that threatened the TARDIS' proper operation. Consequently, the Doctor abandoned him in the year 200,100 aboard a space station orbiting a Dalek-devastated Earth.

Stranded, the former Time Agent had to use his own devices to catch up to the Doctor. He thought it likely that he'd find the Ninth Doctor in the early 21st century, since that was Rose's home era. But his vortex manipulator failed to deliver him there due to the damage caused when he was exterminated by the Daleks, missing by more than a century — and going on the fritz immediately thereafter. This left the immortal no choice but to simply wait from the mid-19th to the early 21st century to reconnect with the Doctor.

During his long wait for the "right Doctor", he experienced what was to him Earth's history first hand. He also had many different relationships, some of which produced offspring. For the majority of those years, he also worked for Torchwood Three. For several decades an informal free-lancer, he eventually became its head in the year 2000. Later that decade, he finally met up with the Doctor, although the Time Lord had by this time regenerated, and was now travelling with Martha Jones.

At the dawn of the 2010s, the 456 returned to Earth. Although Torchwood were able to repel the threat, their success came at the loss of their physical headquarters and painful personal loss for Jack. He therefore disbanded the group and left Earth. However, he returned during the events of the so-called "Miracle Day", and a new Torchwood team arose.

An important facet of Jack's existence on Earth was its temporal complexity. There were whole decades, when multiple versions of Jack existed on Earth. Indeed, during the whole of the 20th century there were always at least two Jacks on Earth, since a younger version was a part of Torchwood Three and an older one was in a grave dug by his brother in ancient Britain. During the World War II era, Earth had at least one more Jack — the younger "con artist" who first met the Doctor and Rose.