Sarah Maitland (Gabrielle Glaister) was a hidden villainess from "Part of a Whole", the penultimate episode of New Tricks' ninth season (airdate October 22, 2012). She was the wife of retired banker Christopher Maitland, as well as a board member of several charities. Christopher was questioned regarding UCOS' investigation into the hit-and-run murder of journalist Simon Bisley, who was working with an intelligence group (which included DCA Robert Strickland and Stephen Fisher) to investigate the IRA for corruption. Strickland had the team re-open the case to solve Bisley's murder after an attempt was made to kill him and Fisher, leading him to believe the corrupt agents were attempting to fully cut loose ends. MI5 was ultimately found to be responsible for the corruption, with a shooting leaving Fisher hospitalized and one of his gunmen dead.
In the episode's climax, Strickland met with Christopher to reveal that Bisley had discovered that the IRA was not only working with convicted drug dealer Carl Dillon, but was also using his crimes to finance arms deals. M15 also gave Dillon instructions to clean up their loose ends once Fisher began investigating the corruption following Bisley's murder at their hand, resulting in the attempts on the members of Strickland and Fisher's intelligence crew. Strickland also revealed that Dillon's righthand man Fisk received calls from someone on where to send his hitmen and was possession of his phone, with Strickland dialling the number of Fisk's cohort revealing their identity: Sarah, who walked into the room as her phone rang cemented her reveal.
When asked how she was recruited by the IRA, the evil Sarah stated that they first took a notice to her when she was attending Cambridge, thus meaning Sarah had been an IRA mole before she met Robert and Christopher and used her connections to them to fulfill her villainous duties to the IRA. Sarah had also been present at Simon Bisley's murder, even pulling his young daughter Ruth out of the way of the car that struck and killed her father. Strickland demanded for the Maitlands to allow him to arrest Dillon for killing Bisley, only for the nefarious couple to refuse and boast that their numerous connections would make it hard to convict them. Strickland countered that the testimony of Ruth and himself could attest to their crimes, showing his willingness to have his career ended by his admission to being part of the intelligence group when Sarah brought it up to him.
Knowing she and her husband were backed into a corner, Sarah agreed to Strickland's terms, allowing Dillon to be arrested and stripped of his IRA protections while she and Christopher made sure any allegations he made against his team couldn't stick. Afterwards, as Strickland predicted, the Maitlands made arrangements to flee town, though the episode ended with a hospitalized Fisher talking about how he hated "loose ends" as a man on a motorbike was shown following the Maitlands' car; indicating that Fisher had arranged for Sarah and Christopher to be killed for their treachery.