Betrayal (1974)

Wealthy widow Helen Mercer hires a young woman, Gretchen Addison, to act as her personal assistant and companion. Unfortunately, Helen is a poor judge of character, as Gretchen is part of a murderous extortionist duo with her boyfriend, Jay. However, Gretchen has second thoughts when she develops genuine affection for Helen. When Gretchen informs Jay that she wants to call off their plot, he refuses and carries on with the plan. Now both Helen and Gretchen may be in grave danger.