A serial killer is operating from his mansion in a remote part of England. His victims are his staff. Every month he employs three people - a housekeeper, cook, and gardener. He sources them from a gang that smuggles in illegal immigrants. The victims don't exist in the eyes of the law. They are invisible. The serial killer is kind to them and gets to know them. Then he kills them in a manner that befits their off-duty interests. Former MI6 officer Ben Sign and former undercover cop Tom Knutsen are alerted to the disappearance of numerous immigrants. Sign suspects they are being murdered by a clever serial killer. But how does he ascertain that person's identity?