"Alfred Hitchcock Presents" John Brown's Body (1956)

John Brown and Company makes furniture and has been a solid business for years. But young Harold Skinner wants to shake things up and make modern furniture. John Brown will have none of it. Soon Harold meets John Brown's wife, an attractive woman in her 30s, who married for money -- though she denies it. Unlike her husband, she's excited about Skinner's modern ideas and wants to help him. Especially after the two of them become lovers. Harold comes up with a "wonderful horrible idea" to get John Brown out of the way: convince him he's losing his mind.