Twenty years ago, Jung I Sin (Go Hyun Jung) lived life as a serial killer after she murdered five men, nicknamed "Mantis" due to female mantises' reputation for eating and killing their mates. She was eventually convicted and imprisoned, leaving behind her son Cha Su Yeol (Jang Dong Yoon), who later became a police officer. Su Yeol has long loathed his mother for her crimes, but when a murder that seems to mimic his mother's tactics takes place, he's forced to enlist I Sin's help to try and catch the culprit. Will I Sin be able to help – or is something even darker afoot? This drama was adapted from the 2017 French series "La Mante."