The last episode of Series Three of BUGS ended with the mysterious disappearance of Ros. This episode takes up the story minutes later.

The Bureau Team begin a frenzied search for Ros, but have few leads. Things don't look good when Ros's bullet ridden car is dragged from a river. There are traces of blood inside the car, and tests prove it to be Ros's.

Ros is given an official memorial service, but, because her body has not been found, Beckett refuses to believe she is dead.

Jan is warned by Army General Russell of the activities of a group of eco-terrorists called Sunstorm. One of the General's own staff, Morag, steals a code-disc from his office, and meets up with one of the activists, Chris. Using the stolen codes, the pair raid Felton Down Genetic Research Centre. They believe genetic engineering is being used here to produce biological warfare weapons. They are unable to penetrate the vault, and leave without discovering any warhead material. They set bombs - which explode as they get back to their van.

In the Bureau, the Team study a fragment of video of the raid. It shows a blurred image of someone who is neither Chris nor Morag. Beckett immediately declares the blurred figure to be Ros. Jan finds the suggestion absurd, reminds Beckett that Ros is dead, and tells him to take leave. There's a confrontation, and Beckett decides to quit the Bureau to search for Ros. Jan appoints Ed as Acting Bureau Chief.

Continuing his search for Ros, Beckett is shocked to discover that she and an old friend, Terry Williams, were founder members of Sunstorm. Ros's mother tells Beckett that Ros and Terry were very close, and had once been engaged to be married. Beckett finds Terry at his electronic gadget shop, Gizmos. Suspecting Terry's involvement in the disappearance of Ros, Beckett places a tracker on his van.

As Beckett leaves Gizmos shop, Morag, Chris and Ros are revealed to be in a back room. Ros is devastated that she can't tell Beckett the truth. She has joined up with her old friend Terry to uncover a conspiracy involving the illegal manufacture of biological weapons. Terry and the others tell Ros that she must remain out of sight. The conspiracy goes deep within the Establishment and not only their safety, but Beckett's too, depends on absolute secrecy.

Ed and Alex are detailed to transport some genetic material from Felton Down to a more secure site.

Ros and the Sunstorm team hack into a government database and discover the planned movement of material, but do not know that the Bureau is involved. They plan to intercept the shipment.

Ed's van is diverted and stopped. Alex are Ed are held up and the genetic material removed from their van. Chris and Terry tell them they have been duped by the authorities: what they are carrying is the material for a genetic weapon. Ros, in the Sunstorm command vehicle, is shocked to discover that Ed and Alex are involved - and even more shocked to see Beckett arriving. Beckett, who has been tracking Terry's van, arrives just in time to prevent Morag, Chris and Terry taking the genetic material.

The sunstormers try to escape from the Bureau team. Ros is desparately torn and wants to explain things to Beckett. Beckett sees Ros in the van. She catches Beckett's eye for a moment but Terry forces her to drive off. Both Ros and Beckett are distraught as they are prevented from meeting.