What is the theme in Suspicion by Dorothy L. Sayers?

The difference between appearances and reality is an important theme in "Suspicion." Mr. Mummery and his wife both have not been feeling well lately. He has been suffering from stomach problems, and she is often tired and sluggish. When Mr. Mummery finds certain suspicious clues, he begins to wonder if their new cook is poisoning them. He finds a can of weed-killer with arsenic in the garage. It is the exact same brand that another cook, Mrs. Andrews, used to poison a family, and it has been opened though he knows he left it capped.