"For Bad Boys Only" is an unconventional detective agency which specializes in helping clients to search for reunite with their first loves. The agency is managed by King Chan (Ekin Cheng), a flirty Casanova, and his partner, Jack Shum (Louis Koo), a perfectionist, with King's younger sister, Queen (Kristy Yang), who has a soft spot for Jack, being the only employee. Recently, Taiwanese tycoon Yung Wing-hing (Ko I-chen) hires the agency to search for his ex-lover, Koon Ching (Shu Qi), whom was Asia's premier female air force pilot. At the same time, a young man named Tin-ngai also hires the agency to search for his missing girlfriend, Shadow (Shu Qi). To King and Jack, Koon Ching and Shadow look strangely identical. Making matters more bizarre, King meets an amnesic girl named Eleven (Shu Qi), who also looks identical to Shadow and Koon Ching. The amorous-natured King also falls in love with Eleven. While searching for Eleven's true identity, King and Eleven were attacked multiple times before Eleven was captured by a group of gun-wielding mysterious men. At the same time, Tin-ngai also finds Shadow, who was disfigured. It turns out that Shadow was captured by Japanese scientist Taro Sakamoto (Mark Cheng), who retrieved DNA from Koon Ching's body and injected into Shadow, hoping to create a female pilot with similar skills. However, the experiment failed and Shadow was disfigured. However, Tin-ngai's love for Shadow did not change and decides to spend the rest of his life with her.
After King and Jack learns of this, they proceed to rescue Eleven. The two of them successfully sneak into Sakamoto's base, where they find dozens of clones that look identical to Eleven, each of them with different mutations. They were all named by numbers, with Eleven being the only perfect experiment with no mutations. However, Sakamoto manipulates Eleven to go mad to kill King and Jack.
影片讲述了Jack与King两人共同开创一间名为“Bad Boy特工”的私家侦探社，专替客人寻找初恋情人的故事。
