Roots: Television charity appeals and campaigns, Survivors, Live After People, 28 Days Later and Dawn of the Dead (the helicopter escape) Thomas Dekker's Patient Grissel "It's all Greek to me
Goofs: How can McAllister's people know the casualty rate of the virus over the last ten years but not the present date?
Dialogue Triumphs: The Doctor: "I'll do my best. It's all anyone can do - it just so happens that my best is... better than most!"
Double Entendres: "This is the beginning of the big one. The turning point for us all, so listen carefully and feel free to get very, very excited!"
"You weren't exaggerating when you said it was going to blow, were you?"
Continuity: The 001 variant Amethyst Icosahedral Plasmic Virus 9007/41 is dormant in every human from Earth, but is extremely vulnerable to the proton particles which are released during the operation of a TARDIS. As a result of the destruction of the Amethyst Viral Containment Station (see: Links) the human population of Earth was infected with single particles of the 001 variant. Projecting a potential mutation of this virus with unknown consequences the Viyrans travelled to Earth intending to wipe out the variant strand using a meson radiation disseminator. Consequentially the virus was not expunged, and the radiation blast triggered a massive chemical imbalance in the human race causing 'the madness', a retrograde condition that stripped away the memories and basic functions of the host and caused the loss of over two billion people, reducing the planet to a scattered population of primitive peoples. In a continued attempt to eradicate the viral strand the Viyrans made contact with Earth, dropping off medication to treat the 'madness' in selected areas and through the Blue Forgotten Planet project - a data capture exercise photographing every human individual on the planet - scanning the population for a definitive analysis of the affected population. The Solent Basin was created ten years ago when an oil rig exploded (possibly the Earth's environmental collapse was well underway)
The Viyrans use a form of "hieroglyphics" (Charley's words) which Charley is unable to understand. They are huge, secretive, and have pincers. Charley notes that they move the laboratories inside their ship around frequently. She describes the interior of their ships as being "like a cathedral built by a plumber, designed by someone who had no taste in colour matching." They continue to use Fratalin's voice, synthesised during Patient Zero, seemingly now their default version for human contact. The Viyrans keep a store of many species from across the universe which they also use in their experiments (the Doctor denounces them as "lab rats"). In three million, four hundred and thirty-six thousand viral outbreaks the Viyrans have 'only' resorted to outright genicode five times.
Mila-Charley has been travelling with the Doctor for a very long time (longer, possibly that Charley did with either of her Doctors), although this is the first trip to Earth they have taken together, despite earlier intentions. Charley and Mila are identical in every way, apart from their memories
Charley was cured of her 009 variant of the Amethyst virus 7001 by the Viyrans and remained with them for several millennia, largely kept in cryogenic storage to be brought out from time to time to 'work' for them on other worlds. In return the Viyrans have promised to reunite her with the Doctor [presumably Charley specified the Sixth Doctor]. It could be inferred therefore that her 'contract' with the Viyrans is now complete and Charley is now free on a restored Earth, the Web of Time being presumably repaired. She still has her TARDIS key. At Charley's request the Viyrans replace a memory of Charley's time with the Doctor with that of 'Mila', now imagined to have black hair and green eyes. Among those adventures is a visit by both of them to Gralista Social.
The Doctor uses the TARDIS to project a temporary "time sphere" - a bubble to effectively trap the Viyrans' disseminator signal in a time loop.He carries a mergin nut (see: Links) which enables him to unlock a chest of medical supplies (but seems useless on a locked door - perhaps it only works on combination locks?)
Links: Patient Zero, Mission of the Viyrans, The Girl Who Never Was, Storm Warning, The Hand of Fear (the Doctor's mergin nut from his TARDIS toolkit). The Doctor recalls a dog he once had and was good friends with ("his bark was worse than his bite") - presumably K9. The Caves of Androzani/The Girl Who Never Was ("You were expecting someone else?")
Location: Earth, including the Solent Basin [a crater created by the explosion of an oil refinery, presumably in the Solent Strait, UK] unknown time - though possibly as little as ten years from the present day), in a closed alternative timeline (see: Continuity)
The Bottom Line: So farewell then, Charlotte Pollard. As a conclusion to the Viyran Strand and the return of Charley Briggs offers more action than resolution, meaning the Doctor plays catch-up for most of the story, and when it comes, Charley's and Mila's revelation (especially Charley's knowledge of the Doctor's future) is a tad understated, swept up in the drama around it. Furthermore, Mila's character is not the semi-stalking psychopath that Patient Zero hinted at her being; after Paper Cuts you might assume she and the Doctor have a good friendship going, but the return of Charley and Mila's ultimate sacrifice make tying this end of the story up a little easy. Which isn't to say the final end is not at least moving, or lacking in good dialogue. Perhaps the last word in the latter Six-and-Charley trilogy is that there are still many open ends and back stories left untold (the origins of the Viyrans, the future of Charley), and some (Amethyst Station, the Dalek Time Controller) which Briggs will return to wrap up the departure of another Eighth Doctor companion.
THE TARDIS DEFENCES
Despite not being an offensive craft, there is ample evidence in the series to indicate that as vehicles of near infinite potential, TARDISes - and the Doctor's Type 40 is no exception, attract a great deal of interest from races and individuals who would seek to use it to their own selfish and destructive ends. It is presumably for this reason that the TARDIS, although pregnable to certain attacks and phenomena (psychic impulses, temporal corrosion, Sirian security drones to name three), is by and large adequately defended from without. It's outer 'shell' has its own integrity field which effectively places anything inside the Ship in a separate dimension. The implication would follow however that, once past this dimensional 'skin', any invading force will in time find its ways into the TARDIS interior, and usually the Console Room. The reason for this may be simple coincidence, or perhaps the Ship's own sentience is sufficient to deduce that any uninvited presence aboard the TARDIS merits the attention (and potentially the problem-solving skills) of the Doctor.
With regard to hostile external environments and situations of course, the most frequently-used automatic defensive technique employed by the Doctor for the TARDIS is the Ship's own evasive Hostile Action Displacement System, or HADS. While generally also effective it is a somewhat randomised feature - in this regard it may be that the similar and indeterminate "time safety jump" the Doctor triggers to escape Anzor's TARDIS in Mission to Magnus is either a connected defensive mechanism for use in the Vortex, or it may simply operate along the same design. For particularly traumatic temporal concussion, the 'Post TARDIS Explosion Stress Disorder System' uses temporal buffers to counteract any trauma, while those inside the Ship are soothed by piped music and a sensation of euphoria (The Song of Megaptera).
Internally the TARDIS has further features as a last line of defence. The questionable merits and effectiveness of the Ships alleged State of Temporal Grace and Isomorphic governance of the TARDIS Console have been discussed elsewhere - in Rat Trap the issue of Temporal Grace is invoked and is patently successful, while in Recorded Time's Chronocide the Volschi believe through a scan of the Doctor's mind that the Doctor's biological presence is necessary for the Ship to work. This indeed sounds not unlike isomorphic control, although the fact remains that in stories before and after this (e.g. A Death in the Family) there's evidence to suggest that this requirement can be worked around. Certainly, it would appear from The Crimes of Thomas Brewster and The Feast of Axos that the Doctor has a further ace up his sleeve in the TARDIS' Sensory Disorientation Forcefield, a harmless, though not strictly inoffensive containment field which works in a localised area and can, presumably, be directed from the Console itself. The least said of A Fix With Sontarans' poison gas jets the better - given that this is the same story where Tegan does not recognise Sontarans, its canonical claim is unreliable at best.