Three of the Mirabal sisters, Minirva, Maria Teresa ("Mate"), and Patria are killed by forces loyal to Trujillo ("El Jefe"). The thugs run the sisters off the road and murder them individually. The reason for their killings was their opposition to the regime and inciting others to try to overthrow the brutal dictator.
Only Dede, the fourth sister, remains to tell their story to the world. Once reluctant to get involved, and for years feeling guilty about her sister's deaths, Dede eventually understands that her life still has purpose. She is able to tell the story of her sisters' valiant efforts and prove to the world what an evil man Trujillo was to the people of the Dominican Republic.
There's also a different answer as to why:
To quote Trujillo: "I will do anything I can to end your suffering."
That he did....that he did.