Whoopee Goldberg stars as a Reno, Nevada showgirl turned Roman Catholic nun in the comedy Sister Act. Deloris Van Cartier is the girlfriend of mobster Vince who witnesses a murder. Her boyfriend finds out what she saw and 0rders his men to kill her. She escapes, and comes under the protection of the police lead by Lt. Eddie Southier (Bill Nunn). He finds refuge for Deloris at St. Katherine's Catholic Church in a rundown neighborhood in San Francisco until it is time for her to testify. The order of nuns is headed by Rev. Mother (Maggie Smith) who gives Deloris the undercover name of Sr. Mary Clarence. None of the other nuns knows who she really is, and thinks she is just a transfer. The Rev. Mother and Deloris are not happy about the situation, but the Monsignor assures Lt Southier Deloris will be safe there.
Meanwhile Sr. Mary Catherine becomes friends with the other nuns. They are enamored with her unorthodox way of doing things. Longing for the old life Sr. Clarence escapes across the street to a bar. She is followed by some of the other nuns. She convinces them that she is spreading the Lord's message to all people regardless of whom or where they are. The Rev. Mother is furious and punishes Sr. Clarence by assigning her to work with the nunnery choir. She develops their voices and eventually they sing for Sunday services. This brings in the neighbors much to the liking of the Monsignor. When the Rev. Mother decides to resign because she feels she is no long respected, Sr. Clarence leaves. She is abducted by Vince's men. Lt Southier finds out there is a leak in his department, and goes to help Deloris. The nuns also learn of Deloris's fate, and fly to Reno to help her. They get word that Pope John Paul II will visit the church. Sr. Clarence prepares the choir to sing the movie hits "I Will Follow Him," "My Guy," and "Hail Holy Queen."