Clark, or "Miss Clark", as Li Cheung called her, was a human woman serving time in an underwater prison pod. She had served two years of her ten year sentence when the Third Doctor and Jo Grant arrived in the pod.
Clark had a plan to escape by starving herself until she was small enough to fit into one of the tiny supply pods that was sent to the prison twice a year. She objected to the Doctor's efforts to release the women from the pod, as it would ruin her plans. She tried to attack the Doctor, but she was stopped by some of the women; the Doctor hypnotised her, calming her.
When the other women used the exercise bikes to provide the power the Doctor needed to free the pod from the ocean floor, Clark once again attacked, this time grabbing Jo by the neck, threatening to kill her if the women didn't stop. The pod suddenly lurched, free of its restraints, and Clark stumbled, releasing Jo. The Doctor struck Clark on the back of her neck, allowing the others to finish the job. (PROSE: Deep Stretch)