Hartigan was as cold-hearted as her Cyberman allies, performing acts of such unspeakable ruthlessness that even the Cyberleader expressed his suprise at her apparent lack of emotion. They promised that she would never be upgraded and that she would be heralded at the court of the Cyber-King, which turned out to be a giant Cyberman hidden under the Thames. However, the Cyberleader betrayed Hartigan and installed her as the mind of the Cyber-King. However, her willpower rewrote the Cyberman software and she took control. Outraged, the Cyberleader attempted to remove her from the Cyber-King but was incinerated.Doctor was in town at this point solving a crisis of a man who thought he was the Doctor because of a backfiring Info-Stamp, which the Cybermen used to store data, in particular data about their enemies. The particular Info-Stamp that backfired contained all the records of the Doctor that the Cybermen possessed and the Doctor himself had to convince the man, Jackson Lake, that he was not the Doctor. The two Doctors noticed the giant Cyberman arising from the Thames and rushed to stop it.
Meanwhile, Hartigan was busy attacking the city when the Doctor contacted her, asking her to surrender. Hartigan refused, so the Doctor cut her Cyber-connection and allowed her to see what she had become. Terrified, Hartigan overloaded the Cyberiad with an emotional backfire and destroyed all the Cybermen bar six, who escaped.
Hartigan herself was apparently obsessed with power and even used a Cybershade (a Cyber-converted Dog) as her butler and carriage driver. Oddly enough, her first name is Mercy, which is ironic since she used to Cybermen to kill hundreds for no apparent reason other than sheer boredom.