I Come with the Rain (2009)

Ex-Los Angeles cop turned private eye Kline travels to Hong Kong in search of Shitao, the missing son of a powerful pharmaceutical conglomerate boss. Enlisting Meng Zi a friend and a former colleague now working for the Hong Kong Police, Kline follows a faint trail left by the ethereal Shitao. The path leads to local gangster Su Dongpo and his beautiful, drug addicted girlfriend Lili. But Kline is distracted from his search, haunted by memories of the serial killer Hasford whose 'body of work' was the reason Kline quit the police force. Will Kline once again need to lose his mind to find his latest quarry?