A new mishnah that establishes some principles about a yevamah... which is established by means of hypothetical examples that can be head-spinning, to be sure... Including principles regarding various sources of prohibition (mitzvah, sanctity). Until the Gemara asks what this principle really adds! Which brings us back to the eilonit, and the implications of those who are connected to her. Also, the widow and how a kohen gadol is prohibited from yibum with her. Part of the question is how far in the process of betrothal, marriage, and consummation took place before the first husband died. Which, eventually, brings the discussion to the conclusion of halitzah.