Cell 114

Cell 114 was the official designation for a species that Jack Harkness was aware of. Very little was known about them, since they didn't usually leave survivors, though he did know that they carried out stealth alien invasions.

Methods and Technology
Cell 114 were ruthless killers. They tended to infiltrate planets, first with an advanced guard of sleeper agents, sometimes watching for years, until they knew all they wanted to know about the species and were ready to take over. Apparently, agents were modified to resemble the native species, though with implants which could take information and transmit it to the main invasion fleet. Active spying was not necessary, the implants constantly took in information from their surroundings. These implants even had built-in weaponry such as spikes and explosives. As well as visually resembling the natives, Cell agents could generate false sensor images as well as a personal force field less than a nanometer around their body (mainly the arm where the implant was stored) for protection. Their electromagnetic fields could disrupt nearby electrical devices such as light bulbs. Sleeper agents were given false memories and artificial personalities so that they believed themselves to be members of the infiltrated species, their real memories stored in the implant until activation.

Their main resource seemed to be the aforementioned implant located in the left arm (at least when mimicking Humans), which contained collected data and transmitted it to the main invasion fleet. They also contained at least one small bomb with which an agent could carry out sabotage or a suicide bombing, or to self-destruct if capture was imminent. Furthermore the arm could be transformed into a sharp spike for close combat. In the case of Beth Halloran, a threat to her life, in the form of a burglar, caused her alien personality to awaken, take over and swiftly kill the intruders. (TW: Sleeper)

Cell 114 attacks tended to be both swift and devastating, requiring only a small number of agents to bring down a planet in most cases due to the comprehensive intel gathered beforehand. Afterward, the planet would be stripped bare. This was described in gruesome detail to Hopkins' team by Iannamet. (WEB: torchwood.org.uk)

History
Torchwood 3, then under Alex Hopkins, was alerted to the existence of Cell 114 in 1998 by an alien named Iannamet, who had tracked a Cell agent to Earth through the Cardiff rift and killed it. The agent had taken on a Human disguise, indicating that there was already a Cell incursion on Earth with which it was attempting to make contact. (WEB: torchwood.org.uk)

The Cell was in the process of infiltrating the Cardiff area on Earth until Torchwood 3 became aware of them in 2008 after inexplicably fierce attacks on two burglars who had invaded the home of one of the agents, Beth Halloran, who thought of herself as an ordinary Human. When Torchwood discovered the Cell 114 presence, other agents were activated and tried to set in motion a plan to explode nuclear missiles stored in an old mine near the city. This attempt was thwarted by Torchwood. However, one of the agents told the team that there were more members of the invasion force already on Earth.