Real Tigers

A man named Paul Lowell, dressed as Spider-Man for a protest, waits on a platform above a London road. He is met by another man in a Batman costume, whom he believes is his protest partner. Instead of joining him, the man attacks Lowell and throws him from the platform to his death.

The narrative shifts to Slough House, an administrative outpost for disgraced MI5 agents known as “slow horses.” Catherine Standish, the office administrator, leaves work and is intercepted by Sean Donovan, a former army lieutenant colonel she knew from her past life as a heavy drinker. Donovan was recently released from prison.

Elsewhere, Marcus Longridge and Shirley Dander are in a pub with their colleague Roderick “Roddy” Ho. Ho expresses a romantic interest in another slow horse, Louisa Guy. Marcus struggles with a gambling addiction, while Shirley uses cocaine. After Shirley slaps Ho for a misogynistic comment, she and Marcus leave. Meanwhile, slow horse River Cartwright visits his former colleague, James “Spider” Webb, who lies in a coma.

Catherine grows paranoid that she is being followed by Donovan and another soldier. After two unanswered calls to her boss, Jackson Lamb, she is ambushed and abducted by Donovan and the soldier, whom she later nicknames “Bailey.” She is taken to a remote farmhouse and imprisoned in a padlocked attic room. Bailey uses her own phone to photograph her while she is handcuffed and gagged. Donovan later visits and asks which of her colleagues she would trust with her life. Catherine gives him a name.

The next morning at Slough House, Catherine’s absence is noted. River receives a text from her phone containing the hostage photograph and a summons to a nearby pedestrian bridge. There, he meets a man in a grey suit, Sylvester “Sly” Monteith, who explains that the kidnapping is a “tiger team” exercise, a simulated attack designed to test MI5 security. He reveals Catherine named River as the person she trusted. Monteith gives River an 80-minute deadline to break into MI5 headquarters at Regent’s Park and photograph the prime minister’s vetting file. Meanwhile, Louisa investigates Catherine’s empty flat and confirms she never made it home.

River bluffs his way into Regent’s Park for a meeting with Diana “Lady Di” Taverner, the Second Desk of Operations, claiming he wants to discuss taking the comatose Spider Webb off life support. In a parallel scene, Taverner meets with MI5 head Dame Ingrid Tearney to discuss Peter Judd, the hostile new home secretary. After his meeting, River proceeds to the archives, where he tries to access the PM’s file but is thwarted by the archivist, Molly Doran. She signals security, and River is apprehended by Nick Duffy, the head of the service’s internal police, known as the “Dogs.” Duffy, wearing a tracksuit that signals a brutal interrogation, questions River, who maintains he was acting to save Catherine.

Back at Slough House, the other slow horses learn that a body was dumped outside a restaurant where Peter Judd was dining. Ho identifies the victim as Sly Monteith, a school friend of Judd’s. River is suddenly released on Tearney’s orders and returns to Slough House, confirming Monteith was the man from the bridge. Lamb then meets secretly with Tearney, who confirms Judd sanctioned a “tiger team” operation run by Monteith’s firm, Black Arrow. She reveals that Monteith’s team, led by Sean Donovan, went rogue, killed Monteith, and now demands the “Grey Books,” MI5’s files on conspiracy theories. Tearney asks Lamb’s unit to handle the exchange on the sly, planning to use the botched operation as leverage against Judd. Lamb agrees, prioritizing Catherine’s rescue.

The slow horses prepare for the mission. Louisa discovers Donovan’s online persona, “BigSeanD,” who is obsessed with weather-control conspiracies, making the demand for the Grey Books seem plausible. Shirley finds the location of the new off-site storage facility, a former Cold War bunker near Hayes. Ho locates the farmhouse where Catherine is being held. Lamb abruptly sacks Marcus for insubordination and Shirley for being high at work, leaving River and Louisa to handle the exchange. Donovan calls River to arrange the rendezvous.

Lamb has Ho drive him to the farmhouse. He incapacitates the guard, Craig Dunn, and rescues Catherine. It is revealed that Craig is the brother of Alison Dunn, the woman Donovan was convicted of killing, and that the other soldier, Ben Traynor, was her fiancé. Meanwhile, Marcus and Shirley, ignoring their dismissal, shadow the operation and follow a Black Arrow van to the facility.

River and Louisa enter the underground bunker and meet the lone guard, Douglas. Donovan and Traynor follow. On the CCTV monitors, they see a large force of Black Arrow operatives swarming the corridors, coordinated by Nick Duffy. Acting on secret orders from Tearney to eliminate everyone involved, Duffy executes Douglas outside. Marcus and Shirley arrive and are ambushed; Marcus is captured, but Shirley escapes.

Inside, the Black Arrow team blows open the archive doors, and a firefight ensues. Ben Traynor is killed, and Donovan is wounded but retrieves the specific file he was truly after. Louisa takes the file from Donovan, who stays behind to cover their escape. Outside, Shirley creates a diversion and frees Marcus just as Duffy is about to execute him. After a brutal fight, River strikes Duffy down with a metal pipe.

In the aftermath, Lamb meets with Taverner. The retrieved file details Project Waterproof, a secret MI5 program overseen by Tearney. Alison Dunn had discovered it and was killed in a staged car crash orchestrated by Taverner years earlier. Donovan’s operation was a revenge scheme manipulated by Taverner. Lamb gives Taverner a decoy folder containing a fishing magazine, secretly keeping the real file for leverage. Later, Lamb reveals to Catherine that her revered former boss, Charles Partner, was a traitor who passed secrets to the Russians. Furious and betrayed, Catherine quits.

In the final scene, Peter Judd’s fixer, Seb, enters Lamb’s office to assassinate him and retrieve the file. Lamb, feigning sleep, ambushes and kills Seb, then calls his slow horses to dispose of the body.