Build a movie plot around this: It's revealed that Miss Madrigal was convicted of murdering who, fifteen years ago? Her step-sister.
The answer to this question is:
An elderly woman hires a governess, Miss Madrigal (Deborah Kerr), with a mysterious past to look after her disturbed, deceitful, and self-centered teenage granddaughter Laurel (Hayley Mills), who has seen off many previous governesses. Laurel feels intense sexual jealousy and resentment of her beautiful mother who lives elsewhere, and has been taught by her Grandmother to hate her mother. When Miss Madrigal arrives, Laurel is intrigued by her apparent lack of a past, and tries to investigate who she is and to "expose" her. Through this investigating, Laurel helps precipitate the climax of the film where it is revealed that Miss Madrigal was convicted of murdering her step-sister 15 years ago and was sentenced to death, though the sentence was commuted and she'd been in prison since then. Instead of running away from this fact once it is revealed, Miss Madrigal uses this painful revelation to convince Laurel and her Grandmother that she was once like Laurel, and that Laurel should leave her Grandmother's toxic environment and go to live with her mother where she can grow into a better person. Laurel understands Miss Madrigal's self-sacrifice as an example of love, and follows her advice to live with her mother.