A mysterious and dangerous race – to the extent that even the Times Lords only had vague details about their origins and history – the Vrall were utterly ruthless, highly effective at disguise, and nearly unstoppable. They appeared to be just a very dangerous alien animal that was used for sport on an abandoned hunting ground on the Moon, but they were actually sentient, although increditsibly cruel, taking great pleasure in tormenting their ‘prey’ that could only partially be attributed to them only being able to eat certain amounts at a time while digesting fresh meat. They were shaped like men, but were about half that height, and could bend and flex themselves to only a hand’s thickness and change colour to blend into their surroundings. They didn’t bleed, and any small cuts or punctures quickly healed over, with one Vrall being shot in the head with a Victorian pistol three times at point-blank range and swiftly recovering.
The most significant thing about their physiology was their powerful digestive enzymes, allowing them to break down their food and easily consume it; when they consumed the brains of sentient beings, these enzymes allowed them to extract memory engrams from their victims and assimilate their knowledge, this ability translating into an ability to transmit those memory engrams in spore form to infuse knowledge into susceptible minds. On a more superficial level, they could also link together to form an even more powerful entity, with their reproduction cycle apparently consisted of the original Vrall splitting into two after consuming a certain amount of matter. The Vrall depicted were able to essentially ‘possess’ humanoids as some form of host, but the precise mechanism of this process is unclear; the only depicted example of this showed a Vrall essentially climbing into the subject’s back after they had been a host for a long time, with internal organs visible through the gap but no sign of a spinal column, suggesting that the Vrall replaced at least some of the subject’s skeleton. The only way to stop one without advanced weaponry was to either completely dismember one (Difficult since they moved so fast) or set off an all-consuming fire (Also difficult, as it would require the destruction of something fairly large to do the necessary damage).
The park The Vrall lived in had been used as a hunting ground for an apparently long-gone empire, populated by deadly beasts and controlled by a creature that was only ever identified as ‘the warden’, with no available information clarifying if it was a natural creature or had been engineered for its current purpose. The atmosphere was controlled by a force field around a large crater, keeping air in but letting solid objects leave, as well as being configured to prevent those species assigned as prey from leaving the park by any meansat some point, The Vrall found human hosts in the form of the Phiadorans, beautiful humanoid women with elfin ears who had once ruled a ruthless matriarchal dictatorship, maintaining control of their subjects through genetically modified pheromonal glands that subverted the will of humanoid males until they were exiled to the park after the empire was overthrown. It was speculated that the only impact this had on the Phiadorans was changing their goal to provide themselves with food rather than an empire, with The Vrall replicating their hosts’ personalities while destroying their free will, keeping the Phiadorans as convenient hosts while they planned their escape.
Aware of Earth’s close proximity, but also aware that the planet was too primitive to reach the Moon on their own in time to be of any use to them, The Vrall eventually attempted an unconventional plan of escape. Since the warden only took action if it or the park was directly threatened, The Vrall created miniature RNA spores encoded with the information necessary to construct a space engine – The Doctor defined the engine as a form of reactionless drive – which they fired out of the dome around the park via a vacuum gun assembled from strips of bamboo and using a naturally-produced nitro-glycerine taken from a local fungus as an explosive. Eventually, one spore struck the mind of a Professor Boyes-Dennison, who managed to develop the engine with the aid of his daughter Emily, working in conjunction with the British Navy (As The Doctor later noted, Victorian society was capable of building a structurally sound spaceship, but simply lacked a propulsion system). Eventually, their efforts produced the three ships of the British Imperial Spacefleet, the Cygnus, the Draco, and the Lynx, which were commanded by Captains Haliwell, Sinclair, and Green, with Haliwell in charge overall.
After arriving on the Moon and discovering the park where The Vrall were trapped, the expedition lost Sub-Lieutenant Granby to what they would later learn was a Vrall, with their subsequent search for him leading to an encounter with the Fifth Doctor and Turlough. The Doctor had discovered a diary of Haliwell’s in the TARDIS time safe (In other words, in his future he would be given the diary and put it in the safe for his past self to find), and had discovered that he and Turlough would be involved in this adventure (Kamelion would have come, but a side effect of the shield prevented his systems from fully functioning in it). Posing as a scientist and his assistant, The Doctor and Turlough helped the expedition find Granby – who had, quite literally, had his brains sucked out, having been attacked by the Vrall to allow them to absorb his knowledge about Earth and its languages. After a narrow escape from some giant spiders, The Doctor and Turlough accompanied the team back to their ships, where Emily and Haliwell were captured by what looked like a traditional flying saucer, and the Professor died of a heart attack. Even more problematic, the Draco had been shot down while trying to get Haliwell and Emily back, and the energy field around the crater had been strengthened to a point that the TARDIS was now immobilised.
While trying to get to the main citadel accompanied by The Doctor and Turlough, the British party encountered one Vrall in Phiadoran form, apparently in danger of falling off a cliff. Turlough saved the woman – who introduced herself as Lytalia, and she claimed that she took knowledge of his language from his mind to explain her having previous spoken in some kind of odd wailing noise which was really just her adapting to English. Taking the team to the Phiadoran camp, the Phiadorans/Vralls presented themselves as victims of a violent coup, requesting aid from the British in shutting down the warden’s citadel in return for being taken to Earth. Aided by the Phiadorans’ knowledge of the crater, the British managed to infiltrate the warden’s base and rescue Halliwell and Emily, culminating in a confrontation with the warden. Able to communicate with the creature thanks to the TARDIS translation circuits, The Doctor and Turlough were able to persuade it to tell them how to shut off the citadel’s defences, the creature apparently worn out from its long time alone and seeking some kind of release (It was unspecified if the warden had been conditioned to follow orders or was simply stuck in a routine).
The British soldiers were then forced to race against time back to their ships, as the force field that kept the air in the crater was shutting down with the death of the warden. However, The Doctor, Turlough, and Lytalia became separated from the rest when their craft was shot down by the citadel’s cannon now stuck on automatic, and only just managed to find their way to the Draco. However, once there they discovered that the helmsman, Stanton, had mutinied after most of the crew had been killed during a failed expedition to the citadel, and only a few crewmen were left. The Doctor managed to get the ship working after repairing damage sustained by the engines in the earlier crash, but he was then kicked out by Stanton’s crew, leaving Turlough alone on the ship as the Vrall/Lytalia decided to abandon her deception and started picking them all off. Fortunately, The Doctor was found by Kamelion, and they managed to rescue Turlough before Stanton drained the air out of the Draco to stop The Vrall after the few remaining crew had been killed. Now reunited with Turlough, The Doctor revealed that he had deduced The Vrall’s true nature while he was recuperating in the TARDIS – his conscious mind being focused on healing and allowing his subconscious suspicionsto take dominance – noting that the Phiadorians identifying the warden’s robots as ‘mechanical servants’ had been the vital clue; since the Phiadorians claimed they took knowledge of the language from Turlough’s mind, they should have referred to them as ‘robots’, while ‘mechanical servants’ was a term that Granby would have used.
Unfortunately, The Doctor, Turlough and Kamelion were unable to stop the remaining ships from landing on Earth, The Doctor having underestimated the time it would take for the ships to reach Earth now that they had more power available while taking off from the Moon than they’d possessed when leaving Earth. With no hope of negotiating with or tricking the Vrall into being taken somewhere else via the TARDIS, The Doctor and Turlough returned to the now-collapsing citadel and recovered weapons from the citadel’s armoury before it was destroyed, returning to Earth and using the weapons to destroy the assembled Vrall. Recognising from Stanton’s reaction to the freedom of his role as an astral pilot that a class-based society like the Victorian era wasn’t ready for space travel, The Doctor had Kamelion impersonate the long-dead Prince Albert to persuade Queen Victoria to abandon the space project. Before leaving, however, The Doctor was given Haliwell’s diary as he knew that The Doctor would be discrete, and he wanted some record of this increditsible adventure to remain. With the last temporal paradox tied up, The Doctor, Turlough and Kamelion departed, leaving Haliwell and Emily to enjoy their upcoming marriage, the two having fallen in love over the course of their time together.