"Line Walker: The Prelude" is a Hong Kong action thriller film released in 2017. It serves as a prequel to the popular 2016 film "Line Walker," focusing on the backstory of undercover police operations. The film explores themes of loyalty, betrayal, and the complexities of undercover work within the Hong Kong Police Force. The plot centers around a group of undercover agents who must navigate dangerous situations while trying to maintain their cover. As they delve deeper into the criminal underworld, they face moral dilemmas and challenges that test their resolve. The film features a cast of notable actors, including Nick Cheung and Louis Koo.