In the long list of the Doctor’s various alien enemies, the Talerians are one of the most distinctive due to their physically vulnerable nature. While most alien races that The Doctor has encountered are virtually immune to bullets due to their technology allowing them to defect the bullets or their natural resilience, the Talerians, although normally dressed in armour plates and about the size of human beings, are shockingly vulnerable to outside attack, their bulbous forms bursting simply when a part of their skin was caught in the wheels of a chair and torn slightly.
The Talerians gained a unique opportunity to expand when they detected a transmission from Earth broadcast from Base Diana, a secret American facility that had been established on the Moon in the 1970s (Contrary to public opinion, the Apollo missions had actually continued after Apollo 17, with Apollos 18 to 22 transporting material to Diana Base until the research staff were able to establish quantum transference technology that allowed them to link Diana Base to a Texas facility so that people could essentially walk from one to the other). At the time, Base Diana was being used for experiments in prisoner rehabilitation by using radiation bursts to ‘purge’ criminals of the memories that prompted their violent tendencies, but the Talerians were able to hi-jack these experiments and use them to transfer Talerian minds into physically stronger human bodies.
Having somehow transferred one of their number into the body of Professor Jackson, the official head of Base Diana, the Talerians attempted to implant further members of their species into the base’s population, but this attack was briefly jeopardised when Liz Didbrook, one of the nurses, realised what was happening and tried to sabotage the quantum displacement equipment. The damage from her attack resulted in the displacement link shifting from a Texas desert to an English park, resulting in at least two deaths by suffocation as people literally walked onto the Moon while a member of the base’s staff walked into a supermarket where the Eleventh Doctor and Amy had just arrived. Tracking the source of the displacement to Diana Base, The Doctor introduced himself as a quantum displacement expert to gain the chance to inspect the base and learn what was happening there, Amy questioning Didbrook – now mentally damaged after a Talerian attempt to take control of her had failed – and gaining some fragmented pieces of the puzzle thanks to her ramblings.
Unfortunately, The Doctor’s attempt to reactivate the quantum displacement backfired when it was sabotaged again after he repaired it, resulting in him being sent back to the Earth end at Hibiscus base in Texas while Amy and the TARDIS were left trapped on the Moon. Although The Doctor was able to return to the Moon using Apollo 23 – the last Apollo rocket, its mission cancelled with the perfection of the quantum displacement method even if it had been kept ready in storage in case it was needed -, he was not in time to prevent Amy being ‘blanked’ by the Talerians after she realised what was happening, her mind wiped clean of all but the basic essentials of information necessary to lure The Doctor into a trap. Despite the odds against him, The Doctor gained an unexpected ally in the form of Major Carlisle, another member of the facility’s staff; while she had been ‘imprinted’ with a Talerian mind, an unexpected power surge while the process was taking place resulted in her retaining control of her body while still being able to access enough Talerian knowledge to pose as one of them.
Discovering that the Talerians retained back-up copies of the human minds they ‘blanked out’ in case they needed to access the human memories later, The Doctor tracked down the liquid storage method used to preserve the back-up copies. When he gave the phial containing Amy’s memories to her blanked body and told her to keep it safe, the small fragment of Amy’s true self left in her body retained enough initiative to prompt her to swallow the liquid, resulting in her true personality asserting itself as the data returned to her system. Inspired by this idea, as Jackson modified the equipment to beam the Talerians into Diana Base in person, The Doctor dispersed the liquid storage system into the sprinkler systems, restoring the humans to their bodies as the few molecules of their true selves present in each part of the sprinkler’s water supply were sufficient to reassert the humans’ true memories, although it took far longer for each human to return to normal due to the comparatively small amount of relevant data present in each drop that landed on each human.
Although the Talerian in Professor Jackson managed to remove the phial containing Jackson’s personality from the storage system before The Doctor could activate the sprinklers, The Doctor was able to slip the contents of the phial into Jackson’s tea – the Talerian having grown used to drinking tea despite starting it simply to maintain the illusion that he was still Jackson -, restoring Jackson to himself. Unfortunately, while The Doctor had been intending to simply force the Talerians to retreat, Jackson, enraged by the guilt of everything that the Talerian had done using his body, shattered a window in his office, committing suicide while destroying all the Talerians due to the rapid shift in internal pressure as the base experienced decompression before the bulkheads could activate.