"Secret Diary of a Call Girl" is adapted from the popular online diary of a sex worker in London, England who calls herself "Belle de Jour". Call girl Belle (Billie Piper) was originally named Hannah. On weekdays, in front of friends and family, she is a girl next door wearing jeans and T-shirts, no different from others. When night comes, she takes out sexy clothes from the closet, puts on chic clothes, and provides professional services to different men. Her life is full of contradictory sides. As a girl, she is caring, cheerful, and extroverted; as a call girl, she is professional, high-end, but not emotional. She carefully and seemingly balances these two sides of her life with ease, and even her best friends don't know her real job. One day, the secret is revealed, the truth is exposed, and the balance is broken. And how will Hannah face the ups and downs of life and emotions?