This is the last part of a pretty long paragraph:

> 
> " ... To my horror I realized that my grandmother had got up out of bed and was coming towards the study. Two second later she opened the door. I will never forget the awful feeling of shame that she ........ (catch) me doing something that she had forbidden me to do."
> 
> 
> 

What is the proper tense to complete the gap: simple past or past perfect? Why?