The title refers to a biological weapon which is created by the beautiful Helen in a government funded laboratory. One night she steals the prototype sample, while her “boyfriend”, Daniel, steals a helicopter. Together they head for an Oil Rig in the North Sea. Unknown to Daniel, vengeance is on Helen’s mind. She believes that the man responsible for her husband Ryan’s death, two years previously, is working on the rig and, with the deadly chemical, she aims to make him pay. Things do not go to plan. The helicopter malfunctions and crashes on a mountain side, badly injuring Daniel. Obsessively driven, Helen must find alternative means to reach the rig without delay. Through murder and deception she abandons Daniel and hi-jacks an army Chinook. By the time Helen and the lone soldier pilot reach the Oil Rig, he has an idea of her plans. In his attempt to take Opaque from her, the phial is lost and Helen is fatally injured. As her life ebbs away on the deck of the rig she sees Ryan and too late she realizes that nothing is what she had believed it to be. Then the phial is accidentally broken and a deadly yellow cloud is unleashed, slowly engulfing the rig. Even a squad of SAS soldiers sent to control the situation cannot stop the deadly Opaque carrying out Helen’s vengeance on almost everyone it touches.