Randall and Hopkirk (Deceased) S01 E13 But What a Sweet Little Room

"Murder Ain't What It Used To Be!"

Jeff investigates the disappearance of a wealthy young heiress's aunt. When she is then killed by a hit and run driver he is led to a medium previously visited by the aunt and uses Jeannie as a decoy to foil a thieving operation in which middle class men rob wealthy widows by murdering them in the room of the episode title, which then transpires to be a gas chamber in disguise.