In this case I imagine the young lady has been taking advantage of her parents being away on a business trip and thinking her grandad will be a pushover. However, he's now caught her coming back in the early morning after overnighting at a party held at her boyfriend's house. Given her manner of dress he's not having much difficulty guessing what she's been up to and threatens to tell her parents. She knows he's not joking and how disastrous that would be. When he offers to keep quiet provided there is no more bad behaviour while they're away she is very relieved and thinks she's got around the old fool. However he's then added there have to be consequences so she'll be punished in the same way he would have punished her mother if she'd every done anything like it. Before she has time to argue her way out of it, her panties are coming down and she realises that she's in for an old fashioned spanking. How humiliating! She just has to hope he won't hit her very hard...