Portrait in Black (1960)

In 'Portrait in Black,' a woman named Sheila conspires with her lover to murder her abusive husband. They plan the murder to look like a natural death, but their plans take a twist when they accidentally kill the wrong person. As they try to dispose of the body and cover their tracks, a mysterious letter arrives, threatening to expose their dark secret. The suspense builds as Sheila's husband's best friend, a general practitioner, becomes suspicious and starts investigating the death. Can Sheila and her lover outsmart the authorities and escape their crime?