It concealed itself in television signals, transferring itself from set to set. It fed off the mental signals of human brains, paralysing them and sucking out their energy with electrical tendrils. This process left the victims mindless and faceless. The victim's consciousness was retained intact, as seen when the Wire showed the Doctor a sampling of its victims on the televisions in Magpie's shop. The Wire used the image of a female BBC continuity announcer to communicate with the outside world. It hoped to use this energy to manifest itself in corporeal form once more.
The Wire stole the face of Mr Magpie, who owned an electrical shop, and gave it back to him, giving him plans for then-future technologies (such as a portable television small enough to carry in one hand) and promised him freedom after the completion of its simple two-phase plan: First, have Magpie sell cheap television sets all over North London so it could feed. Second, have Magpie connect the Wire (inside the portable television) to the television transmission tower at Alexandra Palace on Queen Elizabeth II's coronation day, June 2nd, 1953. Connected to the tower, the Wire would both transmit itself to the estimated three million viewers watching in the vicinity and transmit back energy it would use to reconstitute its corporeal body.
The Tenth Doctor trapped the Wire on a Betamax tape using a homemade video recorder. The Wire's victims were restored to their normal selves. The Doctor seemed confident that the Wire would remain trapped, but said that he would tape over it, just to be safe. (TV: The Idiot's Lantern)
Behind the scenes
The Wire was based on Annette Mills, a children's television host in the 1930s, known to start off her show For The Children, Watch With Mother with the phrase "Are you sitting comfortably? Then I'll begin."
According to the non-narrative sources Doctor Who: Creatures and Demons and Doctor Who: The Ultimate Monster Guide, the Wire escaped to Earth through the communications device of one of its executioners, who was illegally recording the execution. Also, Doctor Who: The Encyclopedia reveals its planet of origin to be Hermethica.