Sleeper agent

Sleeper agents are soldiers used before an invasion. They are often modified to resemble the species that is to be invaded, and are used to gather intelligence on the species and planet.

A specific group of sleeper agents (of which Jack Harkness was aware) was officially known as Cell 114, of which very little was known. The sleeper agents of this species had implants which could take the information and transmit it to the main invasion fleet. Some inplants even had built-in weaponry such as spikes and explosives. (TW: Sleeper)