In 2004 physics professor Philip MacKenna was apparently abducted from a Paris-bound train. His journey was last minute, after a colleague was robbed, and witnesses at the time proved to be using false identities, suggesting a plot to kidnap him for his expertise. Now his wife is getting cryptic emails, impossible to trace, suggesting he is alive, but she has been holding back a secret. A tip off from a Whitehall mandarin leads the team closer to the truth.

The team is less than pleased when arrogant civil servant Stephen Fisher coerces them into investigating the murder - in 1851 - of Abigail Padua, a mathematical expert whose death led to the collapse of her family business and a widespread financial crisis. With no living witnesses and a different geography to the time the team must pore over the original statements to prove that the killer was a witness who lied to the police. However, they, with Strickland's backing, have another mystery on their hands - Fisher's obsession with cracking the case, which itself could have far-reaching effects in the modern business world. Jack however is able to thwart Fisher before he leaves the unit for good to spend his last days in France.

Following an attempt to kill Stephen Fisher, Strickland summons the team to a secret meeting. He tells them that thirty years earlier he and Fisher were part of an intelligence group investigating the IRA but their source journalist Simon Bisley was killed in a hit-and-run and the investigation ended. Now others of the group are being murdered. Bisley had linked the IRA to a career criminal Carl Dillon who, according to Gerry's informant Tinker, has just recruited two assassins and a telling document which could incriminate Dillon was stolen from Bisley's office. However Brian believes that a rogue member of the former secret service team has a vested interest in stealing the document and silencing their colleagues over it, giving Strickland the means to go after them though the team is dissatisfied with the result.