As Dave suspected, Cindy was having an affair with a man named Arnold Barrett, and she had planned to kill him--assumingly to continue the affair and live off her husband's millions. She left to go sailing with Arnold, and gave Nurse Collins the night off so she could go through with her murderous scheme. After returning home, Cindy vented selfishly to Dave about how unfair his situation was to her, while also admitting to the affair and her plans. Following her admission, the evil Cindy unplugged the iron lung and went to the bedroom, leaving her husband near death. She later returned and reactivated the machine, while falsely telling Dave that the doctor didn't want him to be too dependent on the lung.
Cindy later left with Arnold to go swimming, but in a twist ending, Arnold drowned Cindy. It was revealed that Dave was fully aware of his wife's plans to kill him, and he hired Arnold to take down the villainess.