Talerians were the same height as humans, but with grey-green skinned, blobby bodies covered in slimy pustules. Their heads were bulbous, with a slit-shaped mouth full of razor-sharp teeth, and a single, egg-shaped eye. They had no necks. Their skin was rubbery, able to deflect small impacts. More powerful attacks, such as a knife wound, or a change in atmospheric pressure, would rupture the skin, causing the Talerian's liquid innards to gush out, killing them. With each passing generation, the Talerians grew more and more fragile.
The Talerians wielded laser blasters capable of stunning or killing. They also wore armour to protect their frail bodies.
Due to their increasingly fragile nature, the Talerians were a dying race. They grew desperate to find a new home with new bodies and one of them followed an unintentional signal from the mind-altering experiments of Professor Jackson at Base Diana on Earth's Moon. As the man was setting up his equipment for testing, the Talerian managed to enter his body and take over. The Talerian Jackson then began using Jackson's body and equipment to prepare for a Talerian invasion. He began using Jackson's equipment to erase the minds of the Base Diana personnel and replace them with Talerian minds that were being sent at intervals scheduled for Jackson's experiments so as not to draw attention. The Talerian also erased the minds of many of the other people and made them Blanks: humans who were controlled solely by programing with their real personalities shutting off at programed times. The Talerian Jackson kept backup copies of the minds of everyone he erased in the base's data storage and while he kept a copy of the real Jackson, he also kept the real Jackson's personality alive within his mind so he could use his knowledge to blend in perfectly. However, the real Jackson's mind began fading and was eventually barely even there anymore.
By the time the Eleventh Doctor and Amy Pond arrived, most of Base Diana had been compromised by the Talerians. After the Doctor got trapped on Earth briefly, Amy learned the truth and got turned into a Blank. After Amy drank her backup which restored her mind, the Doctor fed the backups of every member of Base Diana into the base's fire suppression system and released the water with the help of Major Carlisle who was able to avoid being controlled by a Talerian due to a power failure at a crucial moment. The Talerian minds possessing the Base Diana personel were destroyed and the original human minds regained control of their bodies. The Blanks were also restored to normal. However, the Talerian invasion was not thwarted as Jackson had kept his own backup on himself and the Talerian mind remained in control. Jackson called in a full Talerian invasion force to take Earth by force, but the Doctor managed to get his hands on Jackson's backup and slip it into his tea. The backup erased the Talerian mind controlling Jackson and restored the mind of the real Jackson. Remembering what he had done under Talerian control and what the Doctor had said about Talerian's being vulnerable to pressure change, Jackson shot out the huge glass window of his office, blowing himself into space. The pressure change through the base killed the Talerian invasion force and the Doctor and Amy were able to block the window and prevent anyone else from being harmed. The Doctor then dismantled Jackson's equipment to keep the Talerians from using it to return. (PROSE: Apollo 23)