A peculiar disappearance case leaves Master Shan (Hui Shiu Hung) mourning the loss of his beloved wife. It also results in Zhang Yi Jing (Samantha Ko) growing up without a mother's love. Master Shan was a locksmith, running a shop called 'Cheng Xin'. However, due to an unexpected incident, he developed dementia. He later takes on a devoted apprentice, Cheng Hao Ren (Philip Keung). Hao Ren inherits Master Shan's exceptional locksmithing skills, becoming a master at opening locks, even surpassing his mentor. Because of his remarkable abilities, he's frequently called upon by the police, and this leads to a friendship with Officer Liu Ye Chen (Raymond Wong). To this day, there hasn't been a lock that Hao Ren couldn't open.

However, during one locksmithing job, he reunites with an old friend, Lee Quan Kai (Chen Puie Kong), whom he hasn't seen in years. From that moment onwards, a series of strange events unfolds. While returning home, Hao Ren unintentionally hears his new neighbour, Miss Guan (Yumi Wong), playing a harmonica with hauntingly familiar melody. Despite never having met Miss Guan, Hao Ren becomes curious and secretly observes her. Simultaneously, through Yi Jing, he learns that the mysterious case surrounding Master Shan's wife's disappearance is far from straightforward and likely connected to Miss Guan's appearance.

As a result, Hao Ren stumbles upon a murder case during one of his locksmithing jobs, making him the prime suspect in Officer Liu's eyes.

What is Hao Ren hiding? Has he encountered a lock he can't crack? What's the chilling harmonica melody about, and how does Quan Kai fit into the puzzle? With Master Shan's wife death, how many people are entangled behind this unbreakable lock? Can Officer Liu help his friend clear his name? Could it be that everyone carries a lock within their hearts, with countless unknown stories waiting behind each door…