When Hong Kong police negotiator Lee Chung-chi (Louis Koo) was informed about the sudden disappearance of his 16-year-old daughter, Lee Wing-chi (Hanna Chan), during her trip to Pattaya, he decides to travel to Thailand to search for her daughter's whereabouts. There, He receives assistance from local Chinese detective, Chui Kit (Wu Yue), and his colleague, Tak (Tony Jaa), and also encounters the American gangster, Sacha (Chris Collins).

When a human head is discovered, the paths of a female student, a cellist, an autistic teenager, a police officer, a prostitute, a teacher and a dog are found to be serendipitously entwined. Who caused the death? What is the motive? Where is the body? Attempts to answer these questions lead to keywords starting with the letter "G". As the case is peeled back layer by layer, the complexity and desperation of the people living in Hong Kong today are slowly uncovered, and a suspenseful multifarious Hong Kong story is revealed.

Veteran detective Cham works with rookie cop Will to hunt down the city's serial killer. The investigation leads them to a lair littered with dismembered female limbs, and an ex-con who killed Cham's wife and child years ago...