The investigation of a murder yields the detectives two witnesses but each one identifies a different man as the killer. Meanwhile, Quinn and Sykes are kept busy by an elderly escape artist practising his dangerous craft in the harbour.
Beneath its surface exists a playground for the murky underworld, a place of illicit rubbish dumps. pollution hazards, the remnants of huge drug hauls, and a graveyard of bodies from years of suicides, accidents and murders. That's the world of the WATER RATS, and elite bunch of 70 brave men and women guarding the harbour against all the elements that threaten to foul the blue waters.