An old woman had control of a boy who could take away someone else's pain. She controlled him by means of a box strapped to his chest that she could activate with a handheld box. She wanted him to use his abilities only for her. She chased him down after he had inadvertently knocked down Nanci Cruz and healed her.
After the woman captured him, Nanci and the Second Doctor tried to rescue him. The Doctor managed to get him away, but the woman captured Nanci. The woman proposed to trade the boy for Nanci, but when she tried to kill Nanci, the boy took away Nanci's pain and channelled it to the woman, who exploded. (PROSE: Mother's Little Helper)