Mona brought the Ninth Doctor to speak to him about the Pierrot that he'd seen. 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, killing 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)