Secrecy

From the very first moment that Phyllida lays eyes on Lucifer, there are secrets. Phyllida is keeping secret from Lucifer the very reason why she is at the crime scene at all. She also keeps secret the fact that she saw the killer's hat in the drawing room and that she, not the killer, is responsible for Lucifer's head injury. Lucifer tries time and again to coax her secrets out of her, and Phyllida tries time and again to coax permission to tell from Mary Anne. This continues until Phyllida is convinced that the necessity to tell overcomes the need to keep her promise.

Also being kept secret are multiple love affairs. Mrs. Fortemain keeps her extramarital affair secret for obvious reasons. Cedric Fortemain and Jocasta Smollet keep their love affair secret for a time to appease Mrs. Fortemain. Like Cedric, Mary Anne's parents do not approve of...