When police officer Christian Kraus attempts to arrest a notorious serial killer who has been cutting a swathe of terror across London, he is unwittingly exposed to something beyond his worst nightmares - a murderous parasitic creature that lives inside a human host.

As the psychological and physical strain of the creature takes hold, Christian finds himself on the run, wanted for murders he did not commit, hunted by an unidentified military organisation, his only hope his best friend and a stranger known as the Professor.

But the vicious organism within him has a terrifying life cycle he knows nothing about, and it threatens not only his own survival, but the survival of millions.