On what basis is one witness believed in this case to permit the woman to remarry? Four answers are brought - the first three try to prove it from Torah laws, but are unsuccessful. In the end, the Gemara concludes that she can marry based on one witness because the rabbis decided to be lenient in order to prevent an aguna situation. That explains why in the end, they are very strict with her if she is wrong - they assume that she will not only rely on the one witness but will do everything she can to make sure that he is really not alive. Rav and Shmuel each state a limitation on the Mishna and the Gemara tries to understand what they each meant by their statements. The Mishna said she needs a get from both husbands - why from the second - wasn't her marriage to him invalid?