The case of a couple who are engaged in sexual intercourse when she realizes she has begun menstruating -- which makes their intimacy a violation of niddah. The question, of course, is what are they to do? With extenuating factors in whether the man is a Torah scholar, for example. Also, some aggadata on the daf: the consequences - on a metaphysical level - if the couple don't separate. And if a couple takes caution with regard to to the time when she expects to start menstruating, they'll be rewarded as a positive consequence.