Sadist Dentist

I can't remember the plot that well but the boy has a fear of dentists because in the past a dentist made the experience really painful. Now, the boy encountered a dentist that makes the treatment painless, which kinda erases the boy's past fear. On another treatment, that dentist's sadist side comes out and makes the treatment very painful for the boy. At that moment, the boy recognized that dentist was the same dentist that gave him the trauma.