Mother Finsey was a woman who was fascinated by the Master's evil, to the extent that she spent some time travelling with him. The Third Doctor and Jo Grant considered her a companion of the Master. Even after the Master tried to kill her once he became tired of her company, Finsey continued to follow his example, even musing that she would have lost her respect for the Master if he hadn't tried to eliminate her.
Following the Master's attempt to kill her, Finsey was left blind, but used specially designed lenses that allowed her to see in infrared when vision was necessary and learned how to cope without them, with these lenses also allowing her to 'imitate' the Master's hypnotic abilities. Her favoured scam was to create false religions and manipulate the converts into giving her all their worldly possessions before she arranged for their deaths in a 'religious event'.
She encountered the Doctor and Jo Grant on the planet Ephros, which was soon to explode in an apparently natural event. In reality, Ephros's destruction was due to Finsey planting rare fire-worms under its surface, which accelerated the planet's natural temperature. As Finsey attempted to escape in a mining cruiser, she was delayed when she had to reprogram the ship's hyperdrive, with the result that she was apparently killed when the Doctor used a transmat to send the fire-worms into space without having the chance to calibrate it to avoid any other form of life. (AUDIO: The Transcendence of Ephros)