Nixon and Turner investigate when two seemingly unconnected victims are attacked with a hammer and left for dead. Having witnessed the first attack, Valentine is forced to cover his tracks after running from the scene following a traumatic flashback, leaving himself and Kapoor looking for a witness who does not exist. When the pair discover that the two victims both undertook contract work for a cancer research laboratory, Turner begins to look at ex-employees of the company with a connection to animal rights groups. Meanwhile, Fletcher and Casper deal with a former boxing champion who has fallen on hard times - and is being blackmailed by the landlord of his local pub.
