In 1996, Prisoner Zero was a captive in an Atraxi extra-dimensional prison. When a crack in the universe opened into Amelia Pond's bedroom wall, Prisoner Zero escaped through to Amelia's house in Leadworth. It hid in a room in Amelia's house, placing a perception filter on the room's door so it would not be found. It hid on Earth for twelve years, watching Amelia grow up. It sometimes ventured into the village, posing as comatose people in Royal Leadworth Hospital whom it had established a psychic link with, a process that took months. If the person it was copying was dreaming about having someone else with them, Prisoner Zero would appear to be multiple lifeforms touching each other. It was actually one creature with many limbs and orifices.
The forms it took were of comatose people. In 2008, Rory Williams, who worked at the hospital as a nurse, took pictures of them with his mobile phone, knowing the people he saw shouldn't be up and about. It was during this time that Prisoner Zero established a psychic link with Amelia (later Amy) Pond.
The Eleventh Doctor belatedly returned to Amelia's house to save her from the multi-form. After Amy had seen its natural form, Prisoner Zero took a shape-shifted form but was confused as to which voice to use for each form. The Atraxi, who had followed the Doctor, broadcast a global warning that they would incinerate "the human residence", Earth, if Prisoner Zero did not surrender. After escaping from Prisoner Zero, the Doctor and Amy went into the village square, where residents of Leadworth were taking pictures of the Sun, obscured because the Atraxi had put a force field around the Earth. The Doctor noticed that Rory was facing the other way, taking a picture of Prisoner Zero, who was disguised as a man with a dog. The Doctor attempted to draw the Atraxi to Prisoner Zero by overloading all the electrical devices in the surrounding area with his extraterrestrial sonic screwdriver, but the screwdriver burnt out and Prisoner Zero escaped again.
Prisoner Zero went to the hospital and killed all the doctors and nurses. It took the form of a woman with two children to fool Amy and Rory, but gave itself away by talking with the woman's voice through a child's mouth. It snarled at Amy and Rory and chased them down the corridors. The Doctor arrived through the window just in time, and revealed his master plan to Prisoner Zero.