She was played by Lindsay Crouse, who later recurred as the villainous Maggie Walsh on Buffy the Vampire Slayer.
Dr. Joan Allenby is a renowned sex therapist and the host of a popular radio show, who is also involved in a relationship with her business partner, David Kincaid. However, when Joan arrlved at her office late one night, she caught David cheating on her with Joan's assistant, Cindy Galt. Heartbroken and furious, Joan turned villainous and sought revenge on David; while also planning to kill him. She began her evil plan by disguising herself as a high-class prostitute dressed in all black, complete with a wig, dress, and high heels. After admiring herself in her disguise, Joan later purchased a gun, and later stashed her disguise in the restroom of a fundraiser she later attends. Joan appears as herself at the gathering, only to head to the bathroom and change into her villainous alter-ego, which she named "Lisa."
Joan snuck out in her disguise and headed to a nearby bar where she planned to meet David. She took him back to the clinic; making sure that she would be seen, and began seducing and enticing him, right before she took out her pistol. At that exact moment, the evil Joan killed David, and later created a scene to make it look like the "Lady in Black" committed the murder. Joan later returned to the fundraiser and changed into her normal clothes, smiling evilly over her vengeful plan succeeding. With Columbo on the case, though, Joan had to divert suspicion from herself. She originally planned to burn her disguise, but she decided to go out as Lisa instead, making sure that she was seen by many people. Joan burned the disguise afterwards, but in the end of the episode, she saw what appeared to be the Lady in Black in her office while Columbo was interrogating her. It actually turned out to be a mannequin in the exact same outfit that Joan purchased. Realizing that she was finally caught, Joan confessed to killing David; stating that she felt humiliated by his infidelity. Though not shown in the episode, Joan was arrested for killing David.