Melissa was a human woman serving time in an underwater prison pod. She was imprisoned for mass fraud. She had served two years of her ten year sentence when the Third Doctor and Jo Grant arrived in the pod. Like the other inmates, she was initially suspicious of them. The Doctor convinced her that he intended to get them out of the pod, though he wouldn't free them.
Melissa took the first shift on the exercise bikes to provide the power the Doctor needed to free the pod from the ocean floor. When the pod floated to the surface, Melissa and the other prisoners saw the sun for the first time in years. (PROSE: Deep Stretch)