Policewoman Yiu Chi-yue (Venus Wong) is a natural born Highly Sensitive Person. She brings chocolate, salty lemon water and a brain with an "on-off switch" with her whenever she investigates cases. Her senses are heightened when she drinks salty lemon water that arouses her keen awareness of clues at the crime scene. Through touching the dead body, gathering evidence and her senses, she empathizes with the victim and reconstructs the sequence of events. Since Chi-yue met restaurant boss Wong Mik-kei (Pakho Chau), the duo are no longer "spectators". Perpetrators at large are killed one after another. Chi-yue covertly challenges some serial killer on several occasions, and their true identity is gradually uncovered. Chi-yue realizes the two of them have conflicting standpoints even though they are like-minded, causing her to face the conundrum of making life choices. In the meantime, Mik-hei astonishingly makes some decision for her.