A woman is sent anonymous emails from someone claiming to know what happened to her husband Phillip Mackenna, a prominent scientist working on cold fusion who disappeared while on a train to Paris. As the team investigate, DAC Strickland receives a warning from Whitehall.

The UCOS team reinvestigates the murder of an unnamed vagrant strangled on a tube train 15 years ago when DNA tests on a suspect in a warehouse robbery reveal him to be the dead man's son.

The team reopen the case of zookeeper Zac Halsey, originally thought to have been mauled to death by a tiger, when blood evidence discovered in his lodgings suggests he was killed before he was found in the tiger's enclosure.

New information links the unexplained death of a market trader to a series of drug rapes in east London - but with that case still ongoing, Ucos are unable to question the chief suspect, so they have to look into their victim's past in their search for clues. Gerry soon finds a likely culprit in the shape of the market inspector who clamped his car, while Sandra has problems of a more personal nature when her hated mother arrives for a visit.

After psychologist Samantha Gerson shows up at UCOS to conduct a study on older men in the workplace, she ends up getting a mixed reception. Samantha also wants a favour and asks Pullman to look at her brother's case. Darren Gerson was the victim of a hit and run in May 2006 which left him with a serious brain injury and complete memory loss of anything about his life before the accident happened. Darren now is convinced that he is still a target because of the package he was carrying at the time.

Sandra's old flame DCI Larson, head of the Met's arts and antiques squad, asks the team to reinvestigate the murder of antiques dealer Mal Baxter. His death was originally thought to be the result of a burglary gone wrong, but new evidence suggests Baxter was a police informant while conducting underhanded deals.

As part of his initiation into a motorbike gang called The Braves, Reece Chapman plans to murder the rival gang member he believes killed his father Eddie, the former leader of The Braves. Desperate to stop Reece, his girlfriend Stephanie Parr turns to UCOS for help in solving Eddie's murder to prove he's targeting the wrong man.

The OAP detectives return to investigate a fresh batch of unsolved crimes, beginning with the murder of a palaeontologist at the Natural History Museum. After a bit of digging, the team discover the victim was a respected but outspoken scientist with a knack for rubbing people up the wrong way - and when they discover he was strongly opposed to the museum's sponsorship deal with a large fuel company, the old dogs soon have plenty of suspects. Amanda Redman stars, with guest appearances from the late Trevor Bannister (Are You Being Served?), Lucy Brown (Primeval) and Natasha Little (Spooks)

The investigation of the murder of Christopher Collins, who was listed on a website as missing by a former employer, meets a dead end until it is revealed that he was in the witness protection programme. Meanwhile, the team wonder what effect the upcoming cuts to the police force will have on UCOS.

The UCOS team are drawn into a world of immigration loopholes, Albanian gangs and family feuds when they reinvestigate the death of an unidentified male discovered on farmland outside Waltham Forest in 1996, in what appeared to be a crucifixion.