Cell 114, their official designation, are a race of invasionary aliens. The arm of Cell 114 uncovered Cardiff were Sleeper Agents, an advance guard to the main force. The Sleeper Agents are tasked with gathering intelligence. To do so, they hide on their target planet, taking on the form of the planet's dominant species, and absorb as much knowledge as they can to aid in the invasion. Sleeper Agents can be stationed in excess of 10 years to aid their integration into society.

To aid their hiding, they are provided with an implant able to gather information subconsciously and protect them against potential attacks. In the case of Earth, the implant is hidden in the arm of the human shell. It provides a blade weapon for use in emergencies, an impenetrable nano-metre thick body shield and a technology mask allowing vital signs to be hidden. When the Sleeper Agents first arrive, their memory is erased and stored in part of the implant to help them maintain their disguise. When the invasion is ready to take place, their memories are reactivated, overriding their "human" personalities and motivations.

The Sleeper Agents in Cardiff were planning to hijack ten nuclear bombs with the aim of devastating the planet and making it easy for their race to take over.

The last Sleeper Agent left alive was asked by Jack when the others were getting to Earth; the dying Agent replied: "They're already here".