[Grammar] Simple past or past perfect

[Mnazar]

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

PS -- I first used the simple past form but the key says that the past perfect form of the verb is correct. Even though it seems okay, I can't understand the reason why it should be in the past perfect.

 

[Matthew Wai]

she ........ (catch) me doing something that she had forbidden me to do.

'Caught' happened after 'had forbidden', so I think the simple past 'caught' is correct.