Although posing as an American volunteering in the Royal Air Force, Jack is actually a former "Time Agent" from the 51st century who left the agency after inexplicably losing two years of his memory.

Before departing, Jack speculates about his immortality and reminisces about his youth on the Boeshane Peninsula, revealing that his nickname had been the "Face of Boe", suggesting that he may one day become the non-humanoid recurring character of the same name, voiced by Struan Rodger.

While Jack manages to repair his friendship with Captain John to some degree, he is forced to place his brother in cryogenic stasis after Gray kills his teammates Toshiko Sato (Naoko Mori) and Owen Harper (Burn Gorman).

[23] Jack subsequently appears alongside the casts of Torchwood and The Sarah Jane Adventures in the two-part crossover finale of the 2008 Doctor Who series, "The Stolen Earth" and "Journey's End".

Civil servant John Frobisher (Peter Capaldi) orders the destruction of Torchwood to cover a conspiracy;[28] in 1965, the British government had authorized Jack to sacrifice twelve children to the 4-5-6, which is shown in flashbacks.

Six months later, having lost his lover, his grandson and his daughter, he bids farewell to Gwen and is transported aboard an alien ship to leave Earth for parts unknown.

[37] Jack's investigations into the miracle repeatedly turn up dead-ends, indicating a decades-old conspiracy to manipulate the global economy, as well as political institutions, for unknown purposes.

The novel also explained that during the events of the Doctor Who episode "Boom Town" (which was set in Cardiff),[3] Jack placed a lockdown on Torchwood activity so as not to create a paradox involving his past self.

The book is composed of fictitious archive notes, personnel forms, photographs, newspaper clippings and staff memos, and offers revelations about the character which would later be confirmed by the television series.

[76] The Torchwood Archives by Gary Russell collects much of this online literature for the first two series in hardback form, including the Captain's Blog section of the BBC America website.

Investigations by a woman named Holly (voice of Eliza Dushku) establish that Jack was kidnapped by the Three Families so they could acquire entire vats of his blood, which she destroys.

[85] Joseph Lidster also wrote a BBC Radio 4 Torchwood drama, "Lost Souls" which aired in Summer 2008 as an Afternoon Play featuring the voices of John Barrowman, Eve Myles, Gareth David-Lloyd and Freema Agyeman.

Set after the events of the 2008 series, Jack and his team make their first international adventure to CERN in Geneva, as part of Radio 4's special celebration of the Large Hadron Collider being switched on.

[86] The special radio episode's plot focuses on the Large Hadron Collider's activation and the doomsday scenario some predicted it might incite, as well as the team's mourning of Toshiko and Owen's recent deaths.

[93] Captain Jack was set to appear in Absent Friends - marking the fiftieth Torchwood monthly range release - which would have seen him reunited with the Tenth Doctor (David Tennant).

[108] John Barrowman describes the character in his initial appearance as "an intergalactic conman" and also a "rogue Time Agent" which he defines as "part of a kind of space CIA" and alludes to the moral ambiguity of having "done something in his past" and not knowing "whether it is good or bad because his memory has been erased".

Furthermore, like Xena, Jack is "a gay basher’s worst nightmare: a queer weapon-wielding, ass-kicking superhero gleefully chewing his way through awesome fight scenes".

[125] From the pilot of Torchwood onwards, Harkness once again wears period military clothes from the second World War, including braces and an officer's wool greatcoat in every appearance.

Julie Gardner describes the coat as "epic and classic and dramatic", while director Brian Kelly believes it gives Jack "a sweep and a presence".

[122] The character is described as both "lethally charming ... good looking and utterly captivating",[100] as well as "flirtatious, cunning, clever and a bit of an action man".

[130] The American political blog Daily Kos states Jack "can certainly be characterised as a Byronic hero, a tragic figure with a streak of melancholy, heroic yet misunderstood, bold yet rash.

Physically, he identifies Jack as dark-haired and strikingly handsome, with masculine physique; he is intelligent and aware of it, to the point of a superiority complex; he demands unquestioning loyalty, has guilty secrets in his past, and is self-sacrificing.

Although Jack ultimately saves the majority of the world's children and finds a way to foil the monstrous 456, the situation in which he is placed forces him to make a morally difficult (and to some viewers, reprehensible) decision.

[130] Davies stated in an interview with SFX that he "loved" the uproarious reaction to Jack's actions, defending the character in saying "He saved every single child in the world!

[166] Simon Brew of Den of Geek praises the dynamic between Gwen and Jack in series four, describing them as "a terrific double act" and states that "Torchwood is at, or near, its best when the two of them are working in tandem".

"[169] Other relationships which have been described or alluded to (both in the television series and other media) include ex-girlfriends Estelle Cole,[170] Duchess Eleanor,[89] Stella Courtney,[90] and Lucia Moretti,[29] ex-boyfriends and Angelo Colasanto,[39] as well as an unnamed ex-wife.

Jim Shelley of the Daily Mirror, in his review of Children of Earth, said "Unlike David Tennant's Doctor, Barrowman's endless appearances on friendly drivel like Tonight's the Night, The Kids Are All Right and Any Dream Will Do, is so over-exposed, 'Captain Jack' is about as intriguing or alien as a Weetabix and twice as irritating.

In a Halloween episode of the 2008 series of American drama Knight Rider, character Billy Morgan (Paul Campbell) dresses up as Captain Jack, whom he refers to as "the time-travelling bisexual".

[193] Jack's notability is largely due to his mainstream representation of a bisexual man in science fiction television, for whom sexual identity is "matter-of-fact",[180] and not an issue.

[98][118][195][196] Channel4.com cites Jack as a positive role model for gay and bisexual teenagers,[10][197] where little had been present for this audience in years gone by and subsequently leading to a greater culture of tolerance.