Yuriko

 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. 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.

When the pod floated to the surface, Yuriko and the other prisoners saw the sun for the first time in years. (PROSE: Deep Stretch)