Captain Jack Harkness is a fictional male human who works for Torchwood Institute in the Torchwood franchise. He has been portrayed on screen by John Barrowman. A charismatic 51st Century time traveller who we first meet in Doctor Who`s episode `The Empty Child` He used to be a Time Agent before turning and becoming a con man. He had been trying to sell a medical capsule to the Doctor, little realising that it contained nanobots, capable of changing the form of anything around it. This includes the dead who were wearing gas masks at the time of their death. After dealing with the robots nightmare, he joins the Doctor on a number of adventures.
At the end of the series, he tackles a number of Daleks and gets killed during the battle to save the satellite from the Daleks. The Doctor and Rose leave Jack behind, believing him to be dead although the Doctor does give the impression he is still alive. He reappears as one of the lead investigators at the Torchwood Institute with a small team of trusted colleagues.
Jack Harkness is unable to die, if he dies, he wakes up again. He is able to live for ever eventually though evolving into a giant head known as the Face of Boe. Jack out lives the planet Earth when the Sun has gone supernova. In the mini-series known as Miracle Day when no-one dies, the only person who is able to die is Jack and must stay alive so that they can solve the puzzle why so many people are staying alive.