Allison Grady (Polly Shannon) is the main villainess from "Double Jeopardy," episode 4.18 of CSI: Miami (airdate March 13, 2006).
Allison Grady is the mistress of Stephen Rowe, who was acquitted of murdering his wife, Melissa. Among other things, it was revealed that Allison had killed Melissa, and both she and Stephen covered it up. Stephen was revealed to be a racist after Nazi propaganda was found on his computer, and he was upset to learn that Melissa was half-Black (on her mother's side which he didn't Know because her father richard being white and her mother being half-black resulted in her being able to pass as fully white Obviously due to clearly being mostly white). After learning that Melissa was pregnant, Stephen had planned on killing Melissa, mainly to prevent the birth of a child who would be partially Black. However Stephen backed out, leaving Allison to do the deed.
The villainess called Melissa to meet her and reveal that she was the one sleeping with Stephen. Shortly after Melissa arrived, Allison stabbed her to death with a knife that she and Stephen used to practice the murder. After Stephen witnessed his girlfriend killing Melissa, the pair worked together to cover their tracks. Allison redressed her and made it look like she was leaving a party, and they dumped her body in the lake. Allison was arrested for killing Melissa Rowe.