How does the dashing Captain Jack Harkness become the Face of Boe in Doctor Who? First introduced during the Christopher Eccleston era, John Barrowman's Captain Jack is a lovable rogue - a Time Agent turned conman that the Doctor manages to put back on the right path. Appearing regularly in Doctor Who during the Russell T. Davies era, Captain Jack eventually earned his own spinoff series, the more mature Torchwood, which ran for four seasons.
Another recurring character during the same period was the Face of Boe, a giant, highly intelligent head floating in a tank, whose advanced age could only be guessed at. Finding an immediate rapport with the Doctor, it soon transpired that the Face of Boe had an intimate knowledge of the Doctor's life, warning him that other Time Lords were in existence. The Face of Boe remained a mystery until the conclusion to Doctor Who season 3, when Harkness casually mentioned that "The Face of Boe" was an old nickname he used to go by. Upon hearing this, the Doctor and Martha mentally join the dots and realize that their giant head friend is actually Captain Jack in the far future.
Jack's transformation can be traced back to Doctor Who's season 1 finale, in which the character is exterminated by a Dalek. Jack's death is undone by Rose Tyler, the Ninth Doctor's assistant, after she accidentally absorbs the Heart of the TARDIS and gains God-like abilities. Far from simply reviving Jack, however, Rose's interference prevents him from ever dying again. Throughout both Doctor Who and Torchwood, Captain Jack is put through a variety of deadly ringers, only to come out on the other side unscathed.
Shortly before unwittingly revealing himself as the Face of Boe, Jack questions the Doctor about immortality, confirming to the audience that while he can no longer die, Jack is continuing to age, albeit at a much slower rate. The Doctor claims to have no idea what will happen, having never come across such a being previously. This exchange strongly implies that, if allowed to age for billions of years, a humanoid could eventually evolve into just a big head, according to Doctor Who mythology. This transformation may be unique to Jack's Boekind heritage, which could slightly differ compared to humans.
Russell T. Davies has always refused to absolutely confirm that Jack Harkness and the Face of Boe are one and the same, but Barrowman and other cast members have referenced the connection as canon. While it's implied that extreme age will result in the loss of one's body, Jack gains several additional abilities through the years, such as higher intelligence and telepathic communication. Jack also falls pregnant, although this trait has also been seen in other males from alien planets.
Several fan theories exist to explain how Jack might've become the Face of Boe. One links to the Headless Monks and Dorian, the blue severed head in a box. Given Jack's swashbuckling ways, it's not unreasonable to think that he was also decapitated at some point during his life, and continued to live from strictly from the neck up. A second suggestion derives from a line of dialogue in "Gridlock," when Brannigan describes a woman who breathed exhaust fumes for 20 minutes and her head swelled to 50 ft. Perhaps Harkness had a similar experience, and the effects didn't register as an injury and therefore didn't heal.
Eventually, of course, the Face of Boe does die, seemingly from a mixture of old age and exhaustion after providing his energy to save New New York in "Gridlock," but the reason Captain Jack suddenly becomes mortal is never addressed. The Doctor describes Jack as a fixed point in time, theoretically meaning he should never cease to exist. Perhaps the effects of the Heart of the TARDIS only had a limited life span, and after billions of years, Jack finally regained his ability to die. Alternatively, maybe the Face of Boe used his psychic abilities to pass on the TARDIS' energy to help rescue New New York, having become more powerful over the course of his lifespan.
Doctor Who season 12 premieres in 2020 on BBC and BBC America.