Mrs Gillyflower discovered a red leech and named it Mr Sweet. She symbiotically lived off Mr Sweet's nectar and used his poison to create the community of Sweetville, where the best of humanity could be sealed away once a rocket inside the empty match factory that contained the poison spread the poison across the atmosphere. Mrs Gillyflower experimented on her daughter, Ada to develop an antitoxin to the red leech poison so she could survive the explosion. She lied to Ada, telling her that she was blinded by her father. She held sermons to invite people into Sweetville.
In 1893, the Eleventh Doctor and Clara entered Sweetville to investigate the "rejects" of Mrs Gillyflower's plans whose skin had turned a bright crimson. The process failed on the Doctor, but thanks to Ada and Jenny Flint, he recovered and rescued Clara from the poison-induced stasis. The Doctor confronted Mrs Gillyflower and she revealed her plan. Clara stopped the rocket from being fired, but Mrs Gillyflower held Ada hostage and ran to the secondary firing mechanism. When the rocket was launched, Jenny and Madame Vastra had already removed the poison from it. Mrs Gillyflower threatened to shoot the Doctor and his friends anyway, but Strax shot at her, and she fell to her death. In her final moments, Mrs Gillyflower pathetically begged her daughter for forgiveness, but Ada refused, to which Mrs Gillyflower expressed pride. (TV: The Crimson Horror)
Mrs Gillyflower was a very intelligent, manipulative and ambitious woman obsessed with perfection. She recruited only what she considered "the brightest and the best" to her cause, while secretly plotting to kill anyone she considered inferior. The Doctor described her as "nuts". She also had a bit of a sense of humour; when the Doctor informed her that her technology could cause a lot of trouble in the wrong hands, she showed him her hands and said, "You know what these are? The wrong hands!"
Mrs Gillyflower was also very abusive, as shown by her use of Ada as a guinea pig in order to create an antidote to make her immune to the poison (blinding Ada in the process) and even defending her actions by calling them "necessary". She seemed to have no affection for her daughter at all, calling her "trivia" and "of no consequence" and even going so far as to take her hostage and leaving no part for her in the new world due to her blindness, which is more cruel considering she was to blame for it. She did, however have an attachment to her red leech Mr Sweet, going so far as to name her own creation, Sweetville, after him. (TV: The Crimson Horror)