Joanna had spent a long period of her life directing First Valley Computing as a successful leader. However, when she reached old age, she found it harder to find the energy for her duties at work. Her unusual exhaustion led her to seek out help from doctors, who discovered her failing heart and gave her little time to live. As her disease progressed, Carew grew weaker and had to leave her position of authority. Joanna remained at home tended to by her niece in lieu of a hospital, which would be of no use at the knowledge doctors could not save her from dying. She knew it put a burden on her niece to care for her when she had children of her own. Carew felt like a child now that she was unable to do anything for herself. Because of this, she checked herself into Ivyday Nursing Home at her own behest to receive 24-hour care.
When Carew's condition hit its lowest point, she lacked the energy to even read, and instead spent her days listening to radio broadcasts late into the night. It was during these broadcasts that Fitzroy first contacted her over the radio, along with several other residents of Ivyday. He promised to rescue her from death under the terms that she helped him settle on the Earth. In order to do this, Carew and the residents who Fitzroy restored from near-death began a plot to revert Earth's technology back to the Dark Ages to remove the electrical interference blocking Fitzroy from emerging onto the planet. Those that refused, such as Rhys Williams's uncle Bryn Williams, had what life was left taken from them. Unwilling to die, she accepted the restoration of her health.
Carew resumed her post at First Valley and secretly began working on a virus to sabotage power stations on a worldwide level. This would disable all technology connected to the stations and allow Fitzroy clearance onto Earth.
Responding to the power failures occurring as the virus was tested, Torchwood Three began working to resolve the issues, leading officer Gwen Cooper to investigate Carew personally. She approached the woman during a midday jog hazily posing as a journalist and enticed Carew to take them to her workplace for further answers. However, Carew quickly saw through the subterfuge and Gwen revealed her connections to Torchwood. Miss Carew grew annoyed with Gwen's prodding and sent her away at the threat of security throwing her out of the building. Gwen returned later that night and broke back into the facility, where she found Carew at work on the virus in privacy. Carew was now sick of her intrusions and used her own strength to wrestle Gwen into submission, then tied her up and held her captive. Rhys, noticing Gwen's unusually long abscence from home, checked her internet history and concluded she would be investigating First Valley. He arrived at the building, found Gwen, and released her, while Jack Harkness and Ianto Jones tracked down Fitzroy's communications from an antique radio. After learning about Fitzroy's plan, they met up with Gwen and Rhys at First Valley.
As Fitzroy directed, Carew climbed to the transmission tower of her building, where she would affix a transmitter to broadcast the virus. Gwen tried to climb up after Carew, which distracted her and caused the elderly woman to fall. She dropped to the ground, suffering fatal injuries. Faintly conscious, Carew remarked she felt she was in a trance for the past few weeks, and it was futile to fight death, as it was a fight no one wins. Gwen tried to comfort the woman in her final moments and keep her from slipping away, but Carew died quietly where she had landed. Fitzroy ceased communications on Earth, having been thwarted by Torchwood, but Jack predicted he would return and try to settle down again. (AUDIO: The Devil and Miss Carew)
Joanna was a reserved woman who did not like to speak about her past. She had outlived all of her lifelong friends and saw growing old as an unpleasant experience. She also preferred to be addressed as "Miss Carew" rather than her personal name. She liked being able to run First Valley Computing, but the tedium of business meetings began to wear on her as she grew ill. After having her life force replenished, Joanna was quick to make sure she remained fit and began vigorous exercise. She loathed to be patronised, one of the reasons she admitted herself into a nursing facility.
Joanna regretted being conscripted by Fitzroy. She considered it like selling her soul to the Devil, but since he had saved her life, she was obligated to serve him.
After receiving new life from Fitzroy, Carew gained incredible strength. She was able to overpower Gwen in a scuffle, causing her to quip that Carew was like a "Supergran".
More prominently than anything, she dreaded death, making her feel helpless. However, when the time came for her to die, she made her peace and accepted it as a necessity. (AUDIO: The Devil and Miss Carew)