Cell 114

Cell 114 was the official designation for a species of which Jack Harkness was aware. Very little was known about them, since they did not 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 advance guard of sleeper agents, sometimes watching for years, until they knew all they wanted to about the planet's dominant species and were ready to take over. Apparently, agents were modified to resemble the native species, 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 a personal force field less than a nanometre 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.

The Cell's main resource seemed to be the aforementioned implant located in the right 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. The arm could be transformed into a large 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. (TV: Sleeper)

History
Cell 114 was in the process of infiltrating the Cardiff area on Earth until Torchwood Three became aware of their presence in 2008. Beth Halloran, who thought of herself as an ordinary human, made inexplicably fierce attacks on two burglars who had invaded her home. After Torchwood discovered the Cell presence, three other agents were activated and tried to set in motion a plan to explode nuclear missiles stored in an old mine near the city. Two of the Cell agents suicide-bombed a military oil pipeline and Cardiff's telecommunications station, and the remaining agent attempted to access the mine's nuclear missiles but was thwarted by Torchwood. However, the agent told the Torchwood team that there were more members of the Cell's invasion force already on Earth, before self-destructing. Beth Hallorman, struggling to cope with the idea of losing her human personality, finally decided to take Gwen Cooper hostage inside the Torchwood Hub. This incited the rest of the team to aim their guns at Beth if she decided to lunge for a kill, and having previously disabled her electromagnetic force field, the bullets would penetrate her. Beth was actually staging a suicide by cop-like death, despite Gwen's pleading to stop. Beth lashed out with deadly force and was promptly shot dead by the rest of Torchwood before she could strike Gwen. (TV: Sleeper)

Torchwood website
The series 2 version of the Torchwood website had a report from Alex Hopkins about Cell 114. On the morning of 27 June 1998, the police had called Torchwood to an incident, considering it "[n]othing they could deal with". They found a grey-haired man, Iannamet, standing over what seemed to be a human body. Iannamet told Torchwood that the man he killed wasn't human, which the autopsies confirmed; it was a member of Cell 114. Iannamet had followed the individual through the rift to 21st century Cardiff, where Iannamet speculated that the Cell 114 agent was trying to contact his "fellows" on Earth, and killed him. Jack Harkness had said he had heard of them, but didn't specify where. Alex Hopkins said that no-one ever got close enough to members of Cell 114 "for a proper study". He did know of their MO — send one agent as an advance guard; disguise the agent as the dominant species with false memories so that even they don't know they're an alien; collect data through their implant; send similarly disguised agents; activate "when the time is right".

What Iannamet knew about the species was that when they strike, they do with "comprehensive information" about the target planet. Iannamet once told of a story where a planet taken on by the cell had started by morning, ended by mid-afternoon, and was stripped bare in a couple of weeks. According to Alex, Iannamet went into such detail about this invasion, that Alex was forced to give his team the rest of the day off, saying, "It’s hard to concentrate in the afternoon when you’ve heard something like that in the morning."