For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
The novel is neatly divided into three parts. The first part/act is about the four Mirabal sisters' coming of age. Each one of them has some sort of crisis, whether it's spiritual, political, or romantic. Important parts of this coming-of-age include going away to boarding school, meeting friends who have lost family members to the dictator Trujillo's assassins, and sexual awakening.
Act II
Part Two is all about the giddy time of the girls coming together for a common cause: overthrowing Trujillo. Three of them, Patria, Minerva, and Mate are tied to their revolutionary missions and are hopeful for the future of their country. They try to convince their fourth sister, Dedé, to join them, but her marital problems keep her from signing up.
Act III
The final act, part three, is the girls' downfall. Minerva and Mate are thrown into prison, then finally released after lots of torture. Then, tragically, Patria, Minerva, and Mate are murdered in the final section of the book. Only Dedé lives to tell the tale.