The Mirabal sisters get the name "The Butterflies" in Chapter Seven. In this chapter, Maria Teresa (Mate) is the narrator, and her story is told in a series of diary entries from 1953 to 1958.

Many important events are mentioned in this chapter: the family's grief over Papa's death, Mate's attendance at college as an architectural student, and Minerva's graduation from law school. By 1957, Mate grieves that, at almost twenty-two years old, she has yet to have a special man in her life. After Minerva's graduation, Minerva asks Mate to help her set up housekeeping in Monte Cristi. Mate agrees to move in with Minerva and Manolo, but she soon discovers that all is not well with the couple.

It seems that Manolo has been staying out late, and there is tension between him and Minerva. Eventually, Minerva admits that Manolo is seeing another woman. As time progresses, however, Mate notices a change in the couple's relationship; this is also about the time that Minerva and Manolo begin attending meetings together. Mate doesn't describe the meetings, but she soon discovers what Minerva has been involved in when she is woken up by a knock on the door one evening.

The stranger who knocked asks for Manolo, but Mate tells him that Manolo is asleep. Seeing that the man is desperate to hide a box, Mate lets him hide the item under her cot. Eventually, the man asks Mate whether she is "Mariposa's little sister." Surprised, Mate answers that she is Minerva's sister. The man then asks, "You aren't one of us, are you?" Mate finds this question cryptic, but after the man leaves, she discovers that there are enough guns in the box to start a revolution.

Eventually, Minerva and Manolo admit to Mate that they are part of the resistance against Trujillo. Mate begs to be allowed to join, and she is accepted into the group with the code name "Mariposa #2." Minerva is "La Primera Mariposa" (the first Butterfly), and later, Patria becomes "Mariposa #3." Together, they are "Los Mariposas" (The Butterflies).