Hello, my partner and I had intercourse while she was on her 4th day through her period cycle. The condom broke and I believe that sperm was inside the vagina but not deep inside. The period blood will push everything out of the vagina but sperm can live inside for about 5 days. She has approx 3 more days left of her cycle and tells me her 2nd day is usually the heaviest. Regardless, should I get plan B just to be safe? If I do within 24hours, the success rate is 90%+ I think?

Lastly, I would like to understand how plan B works. I read a few links stating that it neutralizes the egg so that the sperm has nothing to fertilize but the sperm can live for a while, how long would it take for the egg to come back?