Mona was a woman who, after being forced to flee her home, became a performer in a travelling circus which had settled down in a tiny Nebraska town shortly after the Civil War. She played the Bearded Lady, though her beard was fake, and was in love with Jacob, a fellow employee of the circus who helped unload trunks. By performing in the circus, Mona had travelled most of the United States. She'd also heard rumours of people going missing, and of a figure dressed in white that stalked the fairgrounds.
She met the Ninth Doctor while he was exploring the carnival, and, after reading that he was a maverick working at a sheriff's office on his psychic paper, brought him to speak to Jacob about the Pierrot that he'd seen. She met up with the Doctor later in the evening and joined him in tracking down the Pierrot. After following mismatched footprints to the Big Top tent, Mona was kidnapped by the Pierrot and brought to a trailer full of surgical instruments, when the Doctor arrived and identified it as a Cyberman, which fell through a breach in space-time and had been kidnapping people to replace its rotting human parts.
After the Cyberman attempted to take Mona's heart, the Doctor realised that it had become more human than machine, developing feelings for her, and had taken Jacob's face due to her love of him. The Doctor disabled the Cyberman's emotional inhibitor with his sonic screwdriver, causing it to break down in agony, and rescued Mona. After returning to the carnival and letting Jacob's death sink in, she declined the Doctor's offer to stay with her, as she felt that she should take care of herself, and did not want to take time away from the Doctor rescuing others from the same fate that Jacob had suffered. (PROSE: The Patchwork Pierrot)