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

Another theme in the play is deception and betrayal. By the end of the story, Mr. Mummery is on the brink of the realization that he has been deceived and betrayed. He dearly loves his wife. All this time, however, she has been carrying on an affair with Welbeck. The clues to her affair were available to Mr. Mummery, but he never noticed them. For instance, when the Welbecks come to visit he sees "a relieved glance pass between Ethel and young Welbeck."