Roots: Jules Verne's From the Earth to the Moon, H.G. Wells' The First Men In the Moon, and various 'Planet of Women' B-movies. Emily Boyes-Dennison reads John Stuart Mill.
Goofs: Turlough's thoughts on page 5 are that he "only had a blank space where his past should be" and that he doesn't know where he's from or why he was exiled; this contradicts Planet of Fire. He later tells Lytalia that he is from the 20th century, but doesn't tell her that he's an alien, despite wanting to seem different to the humans.
Continuity: The Vrall are cunning and ruthless predators, that have evolved to be deceitful and cruel. They are thin, six-limbed flexible creatures with vicious claws on the middle set of limbs, and have elastic forms that can stretch or compress. They are about half the size and weight of an average human male. Bullets pass straight through them without causing damage. Their skins are normally dark, but can change colour and texture. They dislike bright light, preferring to hunt by stealth under cover of darkness. Their heads are featureless and sickle-shaped, with the "beak" designed to puncture skulls, since they feed on brains. These beaks secrete digestive enzymes, allowing them to break down the brains. This allows them to extract memory engrams from their victims and assimilate their knowledge. They can parasitically infest a victim by physically hiding inside their bodies, and in doing so they destroy the free will of their victim and can adopt their minds and personalities, thus providing them with a perfect disguise. They reproduce by parthenogenesis and several Vrall can apparently merge to form a single larger creature. Lytalia claims that explosions or fire can destroy them [however she has been taken over by a Vrall and may therefore be lying]. They are susceptible to specially designed energy weapons kept in the Citadel for the use of the hunters, which detect and target them. The TARDIS databank lists them under various names and notes that their origins are uncertain.
The Phiadorans are humanoid, although they have pointed ears and have longer life spans than humans. They are not telepathic. The Phiadoran Clan matriarchy ruled the Phiadoran Directorate systems for many years, employing deception, bribery, kidnapping and assassination to suppress political opposition. The matriarchy members also have genetically engineered pheromone glands, allowing them to exert a potent influence over males, including Turlough (but not the Doctor). The matriarchy was overthrown during the Sarmon revolution, which ended the Directorate, and they were exiled to the hunting park on Earth's Moon, where they remained for over thirty years. During this time they were possessed by the Vrall, and were killed when the Vrall finally emerged from their bodies.
The Phiadorans presumably built the hunting park on the Moon (the Doctor hypothesises that the Clan Matriarchy ordered its construction for their entertainment before they were overthrown). It is contained in the crater on the dark side of the Moon that becomes known as Tsiolkovskii, and is protected by an energy field that maintains its atmosphere. Volcanic vents provide warmth. The hunting park is filled with lethal flora, fauna and fungi, including the Vrall, enormous spider-like creatures, and creatures referred to as night crawlers. The park is controlled from the citadel, an edifice built on a mountain peak in the centre of the crater, and maintained by robots, which non-humanoid aquatic creature known as the Warden controls. In the past, hunters would visit the park and stay in the citadel, travelling out into the park in small flying saucers. The citadel contains an armoury for the hunters to use and various testing facilities for assessing the potential of new prey species. Following the death of the warden, the citadel computers initiate a countdown and also send a hyperspace signal to the park's constructors; when the countdown reaches zero without a reply to the signal, the system causes the volcanic heating vents to overflow, destroying the entire park and citadel beneath a tide of lava and leaving the unusually dark and deep crater reported by Lunik 3.
The first British space ships are launched from a glen in Scotland, with Queen Victoria and John Brown present. They are designed by Professor Boyes-Dennison and work by using impellers to generate an electrical field. The Professor gained the knowledge of how to do this from viral spores launched at the Earth from a crude projectile gun built by the Vrall-controlled Phiadorans on the Moon, in an almost-successful attempt by the Vrall to engineer their rescue. In order to maintain established history, the Doctor has Kamelion impersonate the late Prince Albert and convince Victoria in a visitation not to allow further attempts at space travel. The remains of the two of the three ships that returned to Earth are dismantled and all records of the expedition destroyed, except for Captain Haliwell's diary, which ends up in the TARDIS library.
The Doctor is experienced at moving in low-gravity. He puts himself in trance for twenty minutes, which has the same effect as eight hours sleep would for a human.
Turlough realises that he is still wearing his Brendon School Uniform and wonders why. He considers changing it, but gets distracted. He falls in love with Lytalia, but puts it down to her artificially enhanced pheromone glands. He has read Verne and Wells and the Sherlock Holmes stories. He has also seen martial arts films.
Kamelion can access the TARDIS memory banks, since their data-stream rhythms are compatible. He has difficulty understanding humanoid psychology. His default android form has empathic circuits, which are the means through which he can be controlled, and a pseudometabolic rate, which he can adjust to compensate for different environments. The energy field protecting the crater interferes with his neural functions.
The TARDIS crosses its own temporal wake, meaning that it passes through an area of the vortex where it has recently been or will be. The Doctor confirms that there are no weapons on board. He and Turlough take rucksacks from the TARDIS stores. It contains a time safe, which the Doctor describes as a "permitted paradox". The time safe allows the Doctor to place objects within it and find them at an earlier time, thus warning himself that a dire situation is imminent [the seventh Doctor may have used this to send himself some of those notes from his future]. He tells Turlough that if he doesn't know what he's going to do in the future, he can do what he likes without having to worry about fulfilling predetermined roles.
Links: The Doctor refers to his granddaughter. He tells Turlough about Leela, telling him that Tegan was self-effacing by comparison.
Location: Scotland and the game park in the Tsiolkovskii crater, over several days from 3rd September 1878.
The Bottom Line: Hugely enjoyable homage to the work of Verne and Wells. The notion of Victorian spaceships at first seems too quirky and unrealistic even for Doctor Who, but is explained satisfactorily, and the bloodbath ending is very in keeping with the gloom and violence of season twenty-one.