Roots: The Futurekind may be inspired by Mad Max and the Reapers from Firefly, The setting is reminiscent of the Doctor Who novel City at World's End and the short story Aliens and Predators in Decalog 3: Consequences.
The Doctor alludes to the website Friends Reunited and mentions the San Andreas fault.
Goofs: It's not properly explained how Jack knows this is the right Doctor. [It's implied that the hand is a Doctor detector, but when the Doctor asks, Jack simply refers to the police box.]
How does Jack hang onto the TARDIS? It usually leaves everything behind when it dematerialises, and the time winds (The Enemy of the World) must make it quite hard to hang on in flight.. [It's another side-effect of his resurrection.]
When the Doctor and co come through the gates nobody tries to put the chain back on even though the futurekind are right there. [Maybe they think that firing the gun through the fence will damage it.]
If it's standard practice to check peoples' teeth before letting them into the compound, then how did the futurekind woman get there? [There's speculation that the futurekind is what humans will become if they stay on the planet, maybe she turned after arriving.]
It's unclear how the Doctor knew that Jack was immortal. He didn't see him after The Parting of the Ways, and if he went back to 200,100 AD, then it seems unlikely that he arrived and remained incognito. And it's extremely unlikely that Rose would let him get away with it.
Jack doesn't age. If he arrived in 1869 and didn't realise he was immortal until 1892 (23 years later), why didn't he figure it out in that time? [He may have believed that his resurrection slowed down his ageing, but had no reason to believe that he was immune to death.]
In 100 trillion years, it's pretty much inconceivable that the universe would still have things like planets or people unless it's possible to change fundamental laws of the universe like entropy. [Perhaps this is possible with block transfer computations – see Logopolis. Alternatively, the dates given onscreen are wrong and it's merely 100 billion years into the future.] On a related note, the Doctor says that the stars have all burned up (which would have happened within the first 1 trillion years), but Yana says that the Utopia call came from across the stars.
The Doctor goes from having no idea how any of the future technology works to describing it as “a bit primitive”.
At the end, how exactly does the Master manage to lock the Doctor out of the TARDIS? He appears to simply toggle the lock from the inside, which prevents the TARDIS key from turning. Which isn't how locks work.
Technobabble: “We've accelerated the calculation matrix but it's going to take time to harmonise.”
“ And over here is the footprint impellor system. Now, do you know anything about endtime gravity?”
“But without a stable footprint, you see, we're unable to achieve escape velocity. If only we could harmonise the five impact patterns and unify them, well, we might yet make it. “
We also see a gravitissimal accelerator, a navigation matrix, a boost reversal circuit, Retro feeds, Stet radiation, and a neutralino map which is bound together with gluten extract.
Dialogue Triumphs: The Doctor: Everything's dying now. All the great civilizations have gone. This isn't just night. All the stars have burned up and faded away into nothing.
Martha: Is that what happens, though, seriously? Do you just get bored with us one day and disappear?
Jack: Oh, don't tell him to put his gun down.
The Doctor: He's not my responsibility.
Jack: And I am? Huh, that makes a change.
Yana Oh, it's far beyond the Condensate Wilderness, out towards the Wildlands and the Dark Matter reefs, calling us in. The last of the humans scattered across the night.
Martha: The Doctor sort of travels through time and space and picks people up. God, I make us sound like stray dogs. Maybe we are.
Continuity: Humanity spends a million years evolving into into clouds of gas and another million as downloads, but always returns to the same basic shape.
Malcarisso was inhabited by Chantho's species, the Malmooth, but she is the last of her kind – the only survivor when their conglomeration died. She is happy drinking her own internal milk and believes it is rude not to begin her sentences with “chan” and end them with “tho”. She has been working with Professor Yana for seventeen years,
Martha cannot do shorthand, and knows the location of a medical kit on the TARDIS which includes a stethoscope. The kit is located quite close to the doors.
Rose Tyler is on the list of the people who died in the battle of Canary Wharf (Doomsday).
Not even the Time Lords came this far into the future.
The Doctor can see that Jack is “wrong”, his immortality is a mistake made by Rose – who made him a fixed point in time and space that was never meant to happen, and even the TARDIS reacted against him. Jack says that the Doctor's hand is able to help him detect a version of the Doctor who might know him already.
The Master escaped the Time War by using a Chameleon Arch to disguise himself as Professor Yana in the far future beyond Time Lord influence.
Links: The Doctor mentions refuelling in Cardiff (Boom Town), and he and Martha have a conversation about the events of that story. There are many references to Jack's immortality (seen throughout Torchwood Season 1 and to Rose being trapped in a parallel universe (Doomsday). Jack has the Doctor's hand in a jar (The Christmas Invasion, and multiple episodes of Torchwood). The Doctor notices that the rift has recently been open (Captain Jack Harkness and End of Days).
We see flashbacks to Jack's first resurrection (The Parting of the Ways), the Doctor's fight with the Sycorax The Christmas Invasion), the Face of Boe's final words (Gridlock), and John Smith's watch in Human Nature. When it is opened, the fob watch includes a piece of Roger Delgado's dialogue from The Dæmons and the Ainley Master's laugh.
The Doctor's comment that “not even the Time Lords came this far” is an allusion to both Frontios. Jack borrows the ninth Doctor's catchphrase "fantastic". Yana was found on the Silver Devastation, which is where the Face of Boe is said to have been from in The End of the World. However, given the timespan between the two stories these are almost certainly different places which just happen to have the same name. The exchange about the Doctor being a Doctor of everything is an allusion to The Mutants. Revenge of the Cybermen, and Four to Doomsday.
Extras: This episode has an episode of Doctor Who Confidential,
Location: Cardiff, c.2008 (immediately after the Torchwood episode End of Days); The planet Malcassairo, sometime near the end of the universe (at least 100 trillion AD).
Future History: By this point in the far future, the stars have burned up and faded into nothing, and the galaxies have collapsed. One group of humans took refuge on Malcarisso,and fear the futurekind – a group with different teeth, who they fear are their future. The futurekind hunt humans.
It is over a thousand years since the last University, and time travel is something that is believed to have existed in the old days.
Thousands of years ago, the Science Foundation created the Utopia Project to preserve mankind – finding a way of surviving beyond the collapse of reality itself. Now a message is being sent from beyond the Condensate Wilderness, out towards the Wildlands and the Dark Matter reefs, calling the remaining humans to gather.
Unrecorded Adventures: The Doctor says that he deliberately left Jack behind after the events of The Parting of the Ways, as he knew that he had become immortal. [As Rose would have objected, we should assume that she wasn't present at the time.]
After Jack's first resurrection, he used his vortex manipulator to try and travel to 21st Century Earth to find the Doctor., but ended up in 1869, with the manipulator burnt out. He discovered he was immortal in 1892, after he was shot through the heart in a fight on Ellis Island. He subsequently died in several ways – falling off a cliff, being trampled by horses, dying in both world wars, being poisoned, starved, and hit by a stray javelin.
During the nineties, Jack visited Rose Tyler's estate a couple of times to watch her growing up, but never said hello because he wanted to preserve the timelines.
The Bottom Line: 'The Master reborn.' This is an absolutely brilliant little story. The future world is brilliantly built and interesting, the Doctor's and Jack's reunion is compelling, Professor Yana is brilliant, and Martha and Chantho's friendship is heartwarming. The only downside is that we don't get to see more of Jacobi's brilliant performance as the Master.