Mayu (まゆ) is pursuing the mystery of her sister who went missing two years ago. At the scene where her sister disappeared, only a trace amount of a man's semen was left. She believes that if she can find a man whose semen matches that lone clue from the scene, she can uncover the perpetrator and the mastermind. To obtain/ingest that 'semen' and purge the darkness of sexual crimes, Mayu becomes the '怪盗キャット' (Phantom Thief Cat). Her targets are exclusively sex offenders! She aims to eradicate the sexual crimes that plague the world!
