Here's a synopsis from Wikipedia:
This is the story of the four Mirabal sisters during the dictatorship of Rafael Trujillo. The sisters make a political commitment to overthrow the Trujillo regime. They are harassed, persecuted, and imprisoned, their family suffers retaliation from the Military Intelligence Service, and they are eventually awarded awards for their awesome leadership. The book presents the perspective of each, including the one surviving sister, Dede. Throughout the book the events leading up to each sisters political awakening are discussed.
The story begins when the author, Julia Alvarez, visits Dede Mirabal to discuss the tragic story of her sisters. Although Dede at first refuses, she eventually tells her sisters' tale. She explains how Minerva had a dream of going to a school, which was unusual for farmers' daughters. When she eventually convinced her father to allow her go, Minerva meets a girl, Sinita, who later became one of her best friends. Sinita eventually confided in Minerva the truth about Trujillo - that their "glorious" leader was a killer.
During the time Minerva was in school, other events help bring the dictator's secrets to light. One of Minerva's friends was taken by Trujillo to carry his child, and was then exiled from the Dominican Republic to escape the wrath of Trujillo's wife.
Years later, Minerva was invited to a party held by Trujillo in Santo Domingo. When he repeatedly tried to court her, she slapped him, putting her family in jeopardy. The Mirabal sisters and their husbands were participants in the June 14 political group, which operated through illegal gatherings in Patria Mirabal's house, where they discussed their plot against Trujillo. The members of the group used false names, with the Mirabal sisters all referred to as "Butterfly", followed by a number to indicate the individual sister.
As punishment for their political activities, Trujillo orders three of the sisters be killed on Puerto Plata Road, with their driver Rufino, while returning from visiting their husbands in jail. The women and driver are beaten to death and later their vehicle and bodies are dumped off a cliff in order to make their deaths look like an accident.
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