It is hinted in the episode "Last of the Time Lords" that Captain Jack Harkness eventually becomes the Face of Boe, but when does this occur?

In the Doctor Who episode "Last of the Time Lords," Jack unwittingly reveals himself as the future Face of Boe, but it is not said when this transition occurs. Writer Russel T. Davies previously halted production of comic books or spin-off novels that definitively state the truth or falsehood of the theory, but on March 30, 2020 on Twitter, Davies confirmed that the Face of Boe and Jack Harkness are indeed one and the same.

In series 3, episode 13, Captain Jack faces his immortal existence. He cannot die, but he still ages extremely slowly. He quips to the Doctor and Martha Jones about his attachment to his personal appearance, admitting to vanity. He says he has always been fond of his looks because he was the first person from the Boeshane Peninsula to join the Time Agency and he was so handsome that they slapped his face on posters and dubbed him "the Face of Boe." When the Face of Boe sees the Doctor, he greets him as "old friend," making it clear that the two go way back. The Face of Boe is a humanoid face floating in a liquid, with long undulating tendrils in place of hair. Jack Harkness is played by the rakishly good-looking John Barrowman, but it is possible that over billions of years, Jack evolved into a different kind of being.

Jack's ponderance is a conjecture of what he'd look like if he reached around the age of 1,000,000. At the time of the episode, he was about 2,158 years old due to periods of being buried alive in "Exit Wounds" and then frozen an additional 107 years. As of today, he'd be about 2,177 give or take some overlaps in his timeline. Davies said that it would be entirely realistic for Harkness to look like Boe in 5 billion years after his nose and ears keep growing due to age. Enormous alterations are possible in all that time, and for the immortal traveler, this is how those changes manifest. As to why the Face of Boe has a British accent where Jack Harkness's is American remains a mystery, as does how Jack lost the rest of his body.

With Jack's hypothesis of evolution happening around 1,000,000 years and viewers first seeing the Face of Boe in "The End of the World," 5,000,000,000 years in the future, the final change probably happened somewhere between those two points. How and where he lost his body is up for discussion, but two in-universe explanations seem to be tied together. In the episode "Gridlock," a character reveals that a woman who breathed in the exhaust for twenty minutes experienced swelling of her head to 50 feet. Jack appears in series 12 episode 5, "Fugitive of the Judoon," and he's attacked by nanogenes, the subatomic robots capable of changing someone's body introduced in "The Empty Child," the same episode that introduced Jack. Doctor Who is a show rich with meaning and rife with poetic callbacks, so having Jack physically altered by a combination of these creatures and the exhaust from the place where his existence ends would bring his character full-circle. He says in "Fugitive" that "nanogenes are always after me," so it seems very likely that this attack is the start of his transformation (early in the third decade of the 21st century).

Taking into consideration all the possibilities for Jack's mutation, the most likely reason for his final form is tied to his ultimate resting place. The exhaust in New New York developed sometime between 4,999,999,935 and 5,000,000,053. Jack's head survived the nanogenes, growing several times its original size sometime in those 65 years, transforming Doctor Who's beloved Jack into the Face of Boe.

NEXT: Doctor Who Hints That Captain Jack Is Transforming Into The Face Of Boe