"I Have a Plot" (He Said, She Said)

Recently hubby and I had dinner with my best friend and her husband. My friend and I always wind up chatting about our writing endeavors. Suddenly, her husband said, "I have a plot for a book."

"What is it?" I asked.

Here's the answer. "Two people are in a locked room and no one can get in. One kills the other with a knife carved from ice. When the dead person is found, there is no murder weapon."

My response: "Why did the one person kill the other? What was the motivation?"

"I don't know. It's just a plot."

A plot? Hmmm. People who don't write just don't get it.