The disgraced spies of Slough House faced one of their most personal threats yet in the fourth season of Slow Horses. With Season 5 arriving on September 24, 2025, here is a complete recap of the major twists, tragic losses, and unresolved cliffhangers that set the stage for the new episodes.
The season delivered shocking family revelations, the death of a beloved team member, and a villain so cunning he walked away scot-free. As the status quo at Slough House was shattered, the characters were left more vulnerable than ever, ensuring that the consequences of these events will ripple directly into the new season.
The Shopping Center Bombing and Frank Harkness
The season opened with a devastating event: a suicide bomb attack on a busy shopping center in London . MI5 and the Slow Horses were quickly drawn into the investigation. They initially pursued a man named Robert Winters, who claimed responsibility for the attack .
As the story unfolded, it was revealed that Winters was just one piece of a much larger and more sinister puzzle. The true mastermind was Frank Harkness (Hugo Weaving), a former CIA operative who had been dismissed decades earlier for crimes including embezzlement and torture . Harkness had been executing a long-term, diabolical plan to create his own private army. He did this by fathering numerous sons with different women and then grooming them to become ruthless mercenaries and assassins . These sons, including Winters, were responsible for carrying out terrorist activities on his behalf.
River Cartwright’s Family Secret
The investigation took a deeply personal turn for agent River Cartwright (Jack Lowden). During a trip to France, River discovered the full extent of Harkness’s operation . The most shocking revelation, however, was yet to come.
River learned that Frank Harkness is his biological father . This discovery connected directly back to River’s grandfather, David Cartwright (Jonathan Pryce). It was revealed that David, during his time running MI5, had a history with Harkness. When his daughter (River’s mother) was involved with Harkness in France, David sent a fixer with weapons, cash, and MI5 cold body identities to secure her safe return . This past action would later come back to haunt MI5.
In the season finale, River confronted his father. The reunion was cold and devoid of emotion, with Harkness showing no remorse for his absence and immediately trying to recruit River into his army. River, of course, refused . A dramatic chase ensued, culminating in River apprehending Harkness at St. Pancras train station .
A Major Loss for Slough House
Tragedy struck the team at their home base. Under orders from Frank Harkness, an assassin named Patrice arrived at Slough House with the goal of killing Jackson Lamb . The Slow Horses, minus River and Louisa, fought back to defend their turf.
In the bloody shootout, Marcus Longridge (Kadiff Kirwan) was shot and killed . Marcus was known for his easygoing nature, and his death was a devastating blow to the entire team. His loss is set to have a profound impact, particularly on his close friend Shirley Dander (Aimee-Ffion Edwards), who had shared a strong bond with him and was already grappling with personal struggles .
The aftermath of the attack also revealed a darker side to the newest Slough House recruit, JK Coe (Tom Brooke). After Shirley tried but failed to kill the captured Patrice, Coe unflinchingly finished the job himself . The season also hinted at Coe’s troubled past, revealing he suffers from PTSD after being handcuffed and tortured with an electric carving knife during a previous assignment .
Frank Harkness Walks Free
Despite being in MI5 custody, Frank Harkness proved why he is such a dangerous opponent. During questioning with Diana Taverner (Kristin Scott Thomas), he played his ultimate card .
Harkness revealed that he possessed evidence linking MI5 to the “cold body” identities he had used for years to carry out assassinations and terrorist activities worldwide. This evidence would expose operations done in the name of “Queen and country” . Jackson Lamb presented Diana with letters that detailed how MI5 had used Harkness for their “dirty work” .
To prevent a catastrophic scandal, Diana had no choice but to release Harkness. He walked free from custody, leaving him as a looming threat for the future. As Hugo Weaving hinted about his character, “Frank gets to play his Get Out of Jail Free card… he actually is a survivor and excellently slippery” .
Other Key Developments
Beyond the central plot, several other storylines evolved:
- Catherine Standish’s Absence: Catherine Standish (Saskia Reeves) did not return to Slough House after quitting her position at the end of the previous season. Her absence was felt by the entire team, including Jackson Lamb .
- River’s Grandfather: In a heartbreaking personal moment, River was forced to place his aging grandfather, David, into an assisted living facility due to his worsening memory problems . As River left, David cried out, “You promised me!” leaving River crushed by the decision .
- A Surprising Gesture from Lamb: In a rare show of camaraderie, the typically gruff Jackson Lamb invited River to the pub for a drink after these events, offering a small amount of support .
- Moira Tregorian’s Return: Standish’s replacement, Moira Tregorian (Joanna Scanlan), managed to negotiate her way back to her old job at the Park. She used information uncovered by Roddy Ho about Claude Whelan’s secret “Galahad” identity and his late-night activities to blackmail him into giving her a promotion and a pay rise .
Where Season 4 Left the Characters
The finale of Season 4 left the team at Slough House in a more fractured state than ever before .
- The Team: They are grieving the loss of Marcus and are emotionally shaken. The dynamic of the group has fundamentally changed.
- River Cartwright: He is grappling with the revelation about his father, the guilt over institutionalizing his grandfather, and the knowledge that Harkness is still free.
- Diana Taverner: She remains the Second Desk at MI5, still reporting to the inept Claude Whelan. Her ambition to lead the agency is stronger than ever, and she will likely continue her political maneuvers .
- Frank Harkness: The primary villain is now free and remains a significant threat to national security and to River personally.
Also Read: Slow Horses Season 5 Release Time: When Episodes 1 and 2 Drop in Your Region
