Sister Girl is an exciting, action-packed thriller based on the true story of college-educated freed slave, Mary Beth Bowser. In 1861, Mary Beth has just graduated with honors from college in Philadelphia. It is then that her former mistress, Elizabeth Van Lew, who heads a spy ring in Richmond during the Civil War, recruits her as a spy.

Mary Beth goes undercover to work for Jefferson Davis, the President of the Confederacy. Having studied acting in college, she assumes the identity of a half-wit servant girl named Sister Girl. While Davis confers with his top general, General Beauregard, Sister Girl eavesdrops on the conversation and later copies secret plans which she passes on to the North.

While acting as a go-between between the two men, Beauregard rapes her and she later becomes pregnant with his child. She steals battle plans from him, but he also does not suspect her of being a spy. Sister Girl walks in on Jefferson Davis’s wife, Varina in a passionate embrace with Judah P. Benjamin, the Secretary of War.

Varina bargains with Sister Girl to keep quiet. Later, when Varina finds a secret document hidden in Sister Girl’s hair and vows to send her to jail for spying, Sister Girl threatens to tell President Davis about her affair unless Varina holds her tongue. Sister Girl eavesdrops on a meeting between President Davis and John Wilkes Booth. Booth tries to persuade the President to become part of a plan to kidnap Lincoln. When Davis refuses, Booth vows to go to the Ford Theatre and kill him himself. Upon hearing this threat, Sister Girl rushes to tell Miss Van Lew. However, the visit is interrupted by the local sheriff who suspects Van Lew of harboring Union soldiers, which is in fact true.

He holds a gun to Van Lew’s head and is about to pull the trigger, when Sister Girl tells him the soldiers are in the attic. On his way up the stairs to get the soldiers, Sister Girl shoots and kills him. The next scene finds the Davis’s frantically packing as Grant approaches Richmond. Jefferson Davis is dressed as a woman, hoping to escape unnoticed. At this point, Varina tells him she is going off with her lover, Judah and he is on his own.

A fight ensues. The final scene is a showdown between Sister Girl and Jefferson Davis, when she reveals her true identity to him. Things get physical between them and she pulls out her gun and forces him to give her the contents of his safe, as she says he never paid her. She is ready to kill him, but his pleas for mercy and arguments against violence persuade her to let him go. As he flees the White House, Sister Girl sets fire to the building and she and her baby run off into the night.