Those assigned there are expected to endure dull, routine tasks, along with occasional verbal abuse from their miserable boss, Jackson Lamb, who anticipates that they will leave out of boredom or frustration.
It tells the story of a team of British intelligence agents who have all committed career-ending mistakes, and subsequently work in a dumping ground department of MI5 called Slough House.
After a serious mistake, caused by a colleague but blamed on him, during a live training exercise at Stansted Airport, MI5 agent River Cartwright is demoted to dead-end work at Slough House, a tranche of MI5 reserved for the agency's screw-ups, headed by the alcoholic and impersonable Jackson Lamb.
Lamb assigns River to carry out surveillance on Robert Hobden, a journalist associated with far-right politicians Peter Judd and Roger Simmonds.
River discovers that his colleague and officemate Sidonie "Sid" Baker has also been assigned to Hobden, discreetly copying the contents of his USB flash drive in a café.
Hassan's kidnappers – Moe, Larry, Curly and Zeppo – hold him hostage in a London terrace basement, with it transpiring that Simmonds is behind the kidnapping.
River, having copied the contents of Hobden's USB drive before delivering it to Regent's Park, realises it contains no information and manipulates a rival, James "Spider" Webb, into confirming the same.
Slough House's cyber expert, Roddy Ho, discovers he cannot hack Hobden's laptop as it is air gapped, furthering everybody's suspicions.
Moody returns to Slough House and attacks Min and Louisa when they re-enter, but is killed when he breaks his neck falling down the stairs in a scuffle.
His kidnapping and intended rescue is an unsanctioned false-flag operation masterminded by Taverner herself to potentially increase cooperation with Pakistan as well as neutralise and intimidate the far-right.
It is revealed that Moe (real name Alan Black) was Taverner's mole but was found out and murdered by Curly, who then fled with Zeppo, Larry and Hassan.
Driving out of London, Zeppo and Larry reveal that they never intended to go through with killing Hassan and entertain the idea of releasing him, becoming increasingly concerned with Curly's erratic behaviour and rhetoric.
Lamb and his team flee the property before the "Dogs" (MI5 security personnel) arrive, knowing that since the operation has not gone to plan that Slough House will be blamed.
When the team reconvenes, River recalls taking a photo of Taverner covertly meeting with Black whilst carrying out surveillance training.
River confronts Webb, alleging Taverner had him purposefully feed him incorrect intel during the Stansted exercise so he would fail and be demoted, as he had the photograph of her meeting with Black.
Taverner confronts Judd at his home, revealing that calls he made to his far-right contacts after Hobden's visit resulted in Black's death and the compromise of the mission.
Larry arrives at the sea port of Harwich but is shot dead by the "Dogs" on the orders of Tearney, who plant a weapon on his corpse to justify the killing.
Former field agent Richard Bough, aka Dickie Bow, recognises and follows a man who tortured him during the Cold War, only to die on a rail replacement bus from an apparent heart attack.
Jackson Lamb investigates, finding Bough's hidden phone under the seat with the message "cicada", and gets the team at Slough House to look deeper.
He alleges when stationed in East Berlin that he overheard Popov demanding that the cicada programme remain funded after the fall of the Soviet Union.
River bribes a taxi driver in Stroud, who reveals he drove Chernitsky to an airfield and flying club in Upshott, but was instructed to lie about it and call a number should people come looking.
Newcomer Marcus Longridge replaces Min on the Pashkin assignment and confesses to Louisa he has a gambling addiction, which was the reason he was put in Slough House.
He deduces that Katinsky is the spymaster behind the cicada program (passing himself off as a junior bureaucrat when defecting to avoid suspicion) and was the handler of a traitor in the service.
The website's critical consensus reads, "Slow Horses refreshes the espionage genre by letting its band of snoops be bumbling, with Gary Oldman giving a masterclass in frumpy authority.
The website's critical consensus reads, "Slow Horses' shabby charms reach a full gallop in this superb third season, yielding what might just be Slough House's most compelling operation yet.
The website's critical consensus says, "The stakes get more personal than ever before in Slow Horses' superb fourth season, proving that this spy series is saddled up for the long haul with no signs of fatigue.