Detective Investigation Files is a Hong Kong crime investigation drama series that features five different cases.

Summer 2011: Zhao Zi Hui is in a passionate relationship with her boss Yan Ruo Qiu. Suddenly, her grandma who’s living in Dongbei falls ill and comes to Shanghai to get hospitalized. At the same time, Yan Ruo Qiu’s ex-wife comes back because she wants to have her former husband back. During these tumultuous times, Zi Hui meets Xu Tian in the hospital who suggests her to join hands and open a restaurant together. At first, Zi Hui takes it as a joke, but she slowly starts to see his seriousness and braveness and ends up really opening a restaurant with him. Not only do these two work close together from now on, but their hearts slowly get closer as well

Ren Jie, Zhou Xinlei, and Shen Xiaorou are three girls who were born on the same day but led vastly different lives. On their 22nd birthday, fate brought them together. In an upscale restaurant, Zhou Jianlong, the chairman of Zhou Group, celebrated the 22nd birthday of his step-daughter, Xinlei. Nothing ever worried Xinlei, because no matter what happened, her step-father would be there to take care of it. However, over the course of the dinner, her stepfather suffered a heart attack and died. Little did anyone know that Zhou secretly left a will; while the mansion and $50,000,000 would be given to his wife and step-daughter, all of his company stocks would belong to his missing birth daughter. Upon hearing this, the ambitious executive vice chairman, along with his right-hand man, hired Ren Jie to assume the role of the missing daughter. Using Ren Jie, he could control the entire company. Meanwhile, policewoman Shen Xiaorou was upset because the pervert she caught was released on bail. She went out to search for him and beat him up. Her actions caused her to be reprimanded by her superior. In a fit of rage, she quit her job. Afterwards, she went to Zhou Group to apply as a security guard, and eventually ended up being Ren Jie's bodyguard.

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Kang Hye Mi is an office worker who's been at KS International for 4 years. She used to always be confident and energetic but lost her confidence and her faith in love after witnessing her scum ex-boyfriend cheating on her. To make matters worse, her ex-boyfriend is a general manager at her company, and she can't avoid running into him. Like with all office romances, after breaking up with him, she becomes the topic of all kinds of rumors. The stress and difficulties have been enough for her to constantly consider resignation. One day, she bumps into Na Jung Gook on her way to work and misses the bus, ruining an important presentation. Ironically, Na Jung Gook is joining Kang Hye Mi's team as a new employee, and they become a mentor and mentee. Na Jung Gook is a rare newcomer who is passionate about everything, without avoiding his boss and who lives by maintaining a work-life balance. As Kang Hye Mi works with Na Jung Gook, she finds herself increasingly attracted to him. And the past relationship between Kang Hye Mi and Na Jung Gook is...

Nakatani Setsuko (Matsu Takako) quit her job as a stewardess in order to marry her boyfriend at the time, Hiroshi. But that didn't work out, and so Setsuko's parents, worried that time is passing her by, decide to introduce her to someone using 'Omiai' - a traditional Japanese meeting custom - in the hope she will marry. She meets Hirose Koutaro (Yusuke Santamaria), who is then pressured by his boss to marry Setsuko after a misunderstanding promises to bring shame on their families. Initially, the two are clearly not interested, but gradually, they begin to respect each other.

Seira was raised in India, but she is sent back to Japan to continue her education at an affluent boarding high school. Although she has lived in luxury, the well-bred Seira is kind and generous, earning her many friends at the school. One person who dislikes Seira is the school's director, though she treats Seira well due to the father's fortune. Then, during Seira's 16th birthday party, the director informs her that her father has died, leaving her penniless. As a result, she is forced to work as a servant in order to pay off her school bills. --Tokyograph