In 1974, a young scientific genius Philip Carvel meets an idealistic security services agent, Milner at a secret forum of political, industrial, financial and academic leaders. Milner is in charge of a powerful shadowy organisation known only as The Network. Together they hatch a radical plan they believe will save the human race from the horrific ramifications of over-population...

In the present day, Jessica has been held captive by Milner who has tortured her in a desperate attempt to find out just what adjustment Carvel made to Janus. Ian and Grant are desperately searching for Becky.

The gang find a promising young hacker who discovers the Network's new plan - they are aiming to unleash a weaponized Russian flu that will prompt the world to use their vaccine on V Day. Dugdale finds out that there is Janus in the vaccine and hands it over to the Network. When Anton's drawings and equations are seen by the others, they soon realize who he actually is. Ian is framed by the Network for the passing of his boss. He's growing ever more suspicious of Becky and so doesn't know who he can trust, in his state he phones Milner and tells her Carvel is alive. Milner is preparing to execute Jessica to try and find out if she knows what amendment Carvel made to the Janus.

Dugdale reluctantly agrees to harbour Jessica and after he visits Jen and Alice in a secret prison it's clear he will do whatever The Network asks in order to keep his family safe. Jessica is looking for Ian, as is Milner who believes that he will lead her to Carvel.

After the revelation that her father is alive, Jessica, is determined to find him, with hostage Milner in tow. Arby has takes Anton and Grant to a remote place from his past. Ian and Becky want to tell The Network that Janus is racially selective, so that they can try and stop the flu virus from being released.

In the final episode the race is on to find out who is going to release the flu. The gang track down Dobri Gorski to a police cell, where he tells them the names of the three people he trained to release the flu. A process of elimination leaves them with Terrence, but they still have to find him. Wilson's new ruthlessness is unnerving - can he be trusted? And what will happen when Becky runs out of Thoraxin?.