The season opens with a bomb detonating at Wester's Mall in London, killing forty civilians. The bomber, Robert Winters, posts a video claiming responsibility. When police raid his residence, a booby trap kills three officers. This event puts MI5's new First Desk, Claude Whelan, under intense pressure to manage the fallout.

River Cartwright grows concerned about his grandfather, David Cartwright, a former MI5 legend suffering from declining mental health and paranoia. Louisa encourages River to visit David. When River arrives, a man impersonating River shows up at David's house. David shoots this impostor and then disappears, escalating the mystery around his condition and safety.

Meanwhile, Marcus struggles with a gambling addiction, and the Slough House team sees personnel changes: Standish is replaced by Moira, who proves competent, and JK Coe, who suffers from PTSD, joins the team. The "Dogs," a ruthless MI5 unit, mistakenly believe River is dead after the impostor's shooting. Lamb is asked to identify the body but lies, claiming it is not River. In reality, River is alive and quietly traveling in France, evading capture.

River's disappearance triggers a complex investigation. Lamb, Min, Louisa, and Roddy work to uncover the truth behind the bombing and the threats to David Cartwright. They discover that David has been targeted for assassination, which ties into a larger conspiracy involving MI5's past and present operations.

Taverner, an MI5 officer, confronts Judd at his home, revealing that Judd's calls to far-right contacts after a visit from Hobden led to the death of an agent named Black and compromised a mission. Tearney returns from the US, ordering all loose ends to be tied up after Taverner lies about the operation. Curly, a dangerous antagonist, forces Larry and Hassan to a secluded woodland at gunpoint to carry out a beheading. Larry manages to free Ahmed and attacks Curly with an axe but flees, allowing an injured Curly to recapture Hassan.

Larry reaches the sea port of Harwich but is shot dead by the "Dogs" on Tearney's orders. They plant a weapon on his corpse to justify the killing. Curly prepares to kill Hassan outside what he believes is a Norman castle, which Ahmed reveals is actually a folly. Lamb, River, Min, Louisa, and Roddy track them to this location. A brief shoot-out ensues between Curly and River, ending with Hassan knocking Curly out with a rock. The "Dogs" arrive but the group prevents them from killing Curly, defying orders from First and Second Desk.

River eventually faces a harrowing confrontation with his estranged father, Frank Harkness, the season's main antagonist. During a tense standoff, Frank takes River hostage but escapes by using a grenade as a distraction. A chase follows, culminating in Frank's arrest at a bar after River tracks him down.

In a dramatic twist, Marcus is shot and killed during an ambush orchestrated by Frank's son, Patrice. This loss deeply affects the Slough House team and raises the emotional stakes for River, who has just learned Frank's identity. Patrice is later captured and killed by J.K. Coe, ending the violent conflict.

The season concludes with the implication that Frank Harkness may be released soon due to letters threatening to expose MI5's past collaborations with him. This leaves the future of several characters, including River and the Slough House team, in a precarious state.

The ending of Slow Horses Season 4 concludes with River Cartwright confronting Frank Harkness, the mercenary revealed to be his biological father, and dealing with the fallout of this revelation. The season closes on a quieter, more personal note with River placing his ailing grandfather in a care home and sharing a rare moment of respect and camaraderie with Jackson Lamb over drinks at a pub, signaling a subtle shift in their relationship and setting the stage for future developments.

In the final episode of Slow Horses Season 4, titled "Hello Goodbye," the narrative unfolds with River Cartwright facing the culmination of the season's tension. After a series of dangerous events linked to a covert assassination squad, River is brought face-to-face with Frank Harkness, the enigmatic antagonist who has orchestrated much of the chaos. In a tense encounter at a bar, River reveals that he knows Frank is his father, a fact he had pieced together earlier during his trip to France. Frank confirms this, deepening the personal stakes of the conflict.

Meanwhile, the Slough House team endures a violent attack when Frank's son Patrice, heavily medicated with painkillers, assaults the decrepit Slough House building. Marcus confronts Patrice but is ultimately killed in the struggle, marking a significant loss for the team. Other members, including 'Bad' Sam Chapman, also suffer mortal wounds during the assault.

Parallel to these violent confrontations, River grapples with a deeply personal challenge: his grandfather David's advancing dementia. Despite David's protests and repeated reminders of a promise River made, River makes the difficult decision to place him in an assisted living facility resembling an old Scottish castle. This moment is emotionally charged, highlighting River's internal conflict and sense of responsibility.

The season closes with a poignant scene in a local pub where River and Jackson Lamb share a drink. This is notable as it is the first time Lamb, typically curt and dismissive, shows River a degree of respect by handing him papers to sign--an "account of his movements"--treating him almost like a peer rather than a Slough House reject. The two sit side-by-side, sharing a whiskey, marking a subtle but meaningful shift in their relationship and ending the season on a note of quiet camaraderie and mutual recognition.

In terms of character fates at the end:

• River Cartwright emerges emotionally burdened but more mature, having confronted his father and made hard personal decisions about his family.
• Frank Harkness remains a looming figure, his identity as River's father confirmed, and his mercenary network exposed but not fully dismantled.
• Marcus dies defending Slough House, a significant sacrifice for the team.
• Jackson Lamb shows unexpected softness and respect toward River, hinting at potential character development.
• David Cartwright, River's grandfather, is placed in a care home due to dementia, symbolizing a poignant loss of independence.

This ending balances the high-stakes espionage with intimate personal drama, emphasizing the characters' vulnerabilities and relationships as much as the external threats they face.

Yes, Slow Horses Season 4 has a post-credits scene. After the main events conclude, the final scene shows River Cartwright placing his grandfather, David, into an assisted living facility due to his dementia. David resists emotionally, repeatedly saying, "You promised me!" as River leaves. Following this, River receives a call from Jackson Lamb to meet for a drink at a local pub. In a significant moment, Lamb shows River respect for the first time in the series by handing him papers to sign--an "account of his movements"--treating him more like a peer than a Slough House reject. The scene ends with the two sitting side-by-side, sharing a whiskey as the screen fades to black and the credits roll. This quiet, respectful moment between River and Lamb serves as the post-credits scene, emphasizing a shift in their relationship and River's emotional state.

The five most popular specific questions about Slow Horses Season 4 focusing on plot elements or characters are:

1. Will Season 4 follow the plot of the novel Spook Street, and how closely will it adapt the book's storyline?
2. How will Taverner's new leadership as First Desk at MI5 affect the agency and the Slough House team?
3. What is the significance of River Cartwright's grandfather David's deteriorating mental state, and is he truly suffering from dementia or faking it?
4. What is the nature of River's relationship with his long-lost father, Frank Harkness, and what role does Frank play in the season's events?
5. Why was Frank Harkness, a dangerous mercenary responsible for multiple assassinations, allowed to go free by MI5 despite his crimes?

These questions arise from key plot developments such as the bombing incident, leadership changes, family revelations, and character arcs involving River, David, and Frank, which drive the season's narrative forward.

The TV show Slow Horses, including Season 4 (2024), is not family friendly and is rated TV-MA, indicating it is intended for mature audiences only. It contains strong language, violence, and adult themes that are unsuitable for children or sensitive viewers.

Potentially objectionable or upsetting aspects include:

• Severe profanity: Frequent use of strong language including multiple f-words, s-words, and other harsh insults and slurs throughout the series.
• Violence and gore: Moderate to severe violence with graphic and bloody scenes, including gunshot wounds, blood, and at times shocking and realistic violence.
• Adult themes and mild sexual content: Mild nudity (such as brief underwear scenes), adult toy store visuals, and passionate kissing.
• Alcohol, drugs, and smoking: Moderate to severe depiction of drinking, smoking, and drug use.
• Frightening and intense scenes: Moderate to severe intensity and suspenseful moments that may be disturbing for sensitive viewers.

Overall, Slow Horses is a dark, gritty espionage thriller with mature content that makes it inappropriate for children or those sensitive to strong language, violence, and adult themes.