This episode continues and concludes the story of Episode One.
The Bureau has learned that the genetic material they were transporting from Felton Down is indeed lethal. The material contains a deadly virus that can be targeted, by means of a DNA sample, to attack a specific person.
General Russell enters the Bureau with soldiers and announces that Bureau 2 is now closed. The incredulous Bureau Team are escorted from the premises and Russell takes away the genetic material.
Ros and the Sunstorm members, Terry, Chris and Morag, are still in hiding. Ros phones Beckett, and he comes to see her for a long-awaited reunion. But, unknown to him, his vehicle has been bugged, and Beckett unwittingly leads Russell's men straight to where Ros and the others are.
The General's men attack. Chris and Morag are shot dead. Beckett, Ros and Terry escape just as the building they are in erupts into flames. Hospitalised, Ros tells Beckett the true events of her "abduction" at the end of Series Three. In flashback, we see how Terry came to Ros's flat and begged for her help in dealing with the genetic weapon conspiracy. Terry was being pursued by the General's men, so Terry and Ros were forced to flee from the flat minutes before Beckett and the others arrived. Ros and Terry's car was pursued, and Ros was wounded by broken glass. The car crashed into a river. She and Terry managed to swim ashore, evading the General's men. She then took advantage of the fact that she had been presumed dead.
Beckett is angry that Ros did not tell him what was going on. He is also jealous that she chose to become so involved with her old love, Terry.
Meanwhile Ed, Alex and Jan have been gathering information on General Russell. Jan learns from Simon, an army psychiatrist and old friend, that Russell had a breakdown after the death of his wife. She further discovers that his children died on the same day as his wife. Their gravestone bears the words: "Sacrifices on the altar of science."
Ed and Alex intervene as Russell's men attack a visiting scientist, Andreas Yerevenkian, at his hotel. Yerevenkian is giving a deposition to the Ethics in Science Commission. Yerevenkian is not harmed, but his suitcase is stolen.
Beckett, Ed and Alex break into the closed-down Bureau to uncover the truth about Russell's background. They find that his family where killed when a small quantity of experimental virus escaped from a genetic research lab run by Yerevenkian.
Russell's men visit Felton Down research lab and persuade Dr Coreoran to bond a sample of Yerevenkian's DNA to the virus, using a hair from Yerevenkian's hairbrush. Thus they will target the virus to kill only Yerevenkian. Later, Terry and Ros arrive at Felton Down to meet Terry's contact, and discover that Corcoran had duped Russell's men. He has not bonded the DNA to the virus. Rather than just killing one person, the virus will kill anybody who comes into contact with it.
The BUGS Team realise that Russell has been the instigator of the conspiracy all along. He has used Corcoran at Felton Down to manufacture the genetically engineered virus in order that he, Russell, may make a public attack on Yerevenkian and get revenge for the death of his family, thus turning Yerevenkian's technology against him. They realise that the Ethics in Science Commission would be the ideal public forum for Russell's attack.
Russell plants the virus in the air-conditioning system at the conference centre where the Commission is meeting. As the air-conditioning activates and the temperature in the ducting drops, the lethal dose will be released.
Beckett, Ed and Alex arrive at the conference centre. Whilst Ed and Alex search for the virus Beckett moves into the main chamber to track Russell. Russell interrupts Yerevenkian's deposition and tells him that he has planned to kill him for causing the death of his family: Russell's view is that all experiments in genetic engineering are too dangerous and should be stopped. Yerevenkian expresses remorse for the death of Russell's family but argues the benefits of genetic experiments. Ed and Alex disengage the genetic material just before it is about to be released. Beckett confronts Russell and a fight develops. Ros and Terry arrive and are drawn into the conflict with Russell and his men. In the fighting, Terry is killed and Ros is distraught at the loss of her old friend.