In the opening sequence of Slow Horses Season 3, the story begins in Istanbul where Sean Donovan pursues his lover Alison Dunn, who plans to leak a critical MI5 document to the press. Alison is killed in what is staged as a suicide, and Sean fails to save her in time. This event sets the tone for the season and introduces Sean as a key antagonist with a personal vendetta.

One year later, Slough House is shown as a neglected department filled with old MI5 files. Catherine Standish, the office manager, and River Cartwright argue about the importance of these files. That night, Catherine is kidnapped by Sean Donovan and his team, initiating the main conflict of the season. Sean's team is revealed to be a "Tiger Team," a covert squad hired to test MI5's vulnerabilities, but Sean goes rogue with his own agenda.

Following Catherine's abduction, Slough House's team, led by Jackson Lamb, mobilizes to rescue her. Lamb delegates tasks and begins to realize how much they rely on Catherine's calming presence. Meanwhile, River receives a photo text showing Catherine with a gun to her head and a mysterious phone call to meet a former MI5 agent named James "Spider" Webb. To secure Catherine's release, River must retrieve a top-secret file, escalating the stakes.

Shirley and Marcus break into Sean's flat to uncover his plans. They discover Sean intends to break into the Grey Books, which contain the most classified secrets of the British government and allied countries. Sean refuses to release Catherine as per the original instructions, confirming his rogue status. This revelation shifts the mission from a rescue to preventing a catastrophic leak of intelligence.

Sean and his team attempt to escape with Catherine and the stolen file. During their escape, Sean is shot but manages to pass the file to Louisa, instructing her to leak it to the press. They part ways with Sean, leaving him for dead. Sean fights off Chieftain soldiers but is ultimately killed by their leader, Fenwick. Louisa and Sean try to exit but find the area heavily guarded, preventing their escape.

Jackson Lamb arrives at a farm where Catherine is held. He sets traps to defend against pursuers Hobbs and Sturges. Lamb kills Hobbs, and Sarah kills Sturges on the stairs. Roddy arrives late and drives a double-decker bus into the farm, but the situation has already been resolved by then. This sequence is a tense, step-by-step confrontation with clear actions taken by each character to protect Catherine and neutralize threats.

Throughout the season, the team at Slough House works to track Sean and prevent the exposure of the Grey Books. They face internal struggles and betrayals, with Lamb orchestrating their moves carefully. The season builds to a climax where the team confronts Sean's plans and the broader implications of his rogue operation, emphasizing the high stakes of espionage and loyalty within MI5.

The season ends with the resolution of Catherine's kidnapping and the prevention of the Grey Books' exposure, but it leaves open the consequences of Sean's actions and the internal challenges Slough House faces moving forward. The final scenes underscore the precarious balance of power and trust within the intelligence community.

The ending of Slow Horses season 3 sees the Slough House agents narrowly escaping a deadly assault by MI5 operatives sent to eliminate them and destroy incriminating files. Marcus kills the antagonist Duffy in a brutal fight, and River and Louisa manage to get the secret file out of the facility. River's grandfather, David Cartwright, burns the original file to protect MI5's reputation, but River secretly leaks a copy, leading to the resignations of MI5's First Desk Ingrid Tearney and the Home Secretary Judd. All the main Slough House agents survive, though Sean Donovan, the rogue operative, is killed.

In the final episode of Slow Horses season 3, the tension reaches a peak as multiple storylines converge. Catherine Standish and Jackson Lamb are holed up in a house, under threat from two of MI5's First Desk operatives sent by Ingrid Tearney to silence them. Meanwhile, River and Louisa are trapped inside The Facility warehouse, where they are under heavy fire from Tearney's armed agents, including her lead enforcer Nick Duffy.

The secret file they are trying to retrieve is revealed to be of immense importance. Sean Donovan, a former head of security at the British embassy in Istanbul, had kidnapped Standish and coerced River and Louisa to help him infiltrate The Facility to find this file. The file contains evidence of a failed operation in Istanbul and incriminating information about Tearney's involvement in covering it up, including the death of Donovan's girlfriend, MI5 agent Alison Dunn.

As the assault intensifies, Marcus engages in a brutal hand-to-hand fight with Duffy. The fight is fierce and desperate, ending with Duffy pulling a knife. Louisa intervenes by striking Duffy with a concrete block, killing him instantly. This violent confrontation neutralizes the immediate threat to the Slough House team.

Inside the facility, River and Louisa, with the help of Shirley and Marcus, manage to evade the heavily armed operatives. They navigate through the facility, fighting off attackers and reaching an exit hatch just in time. Shirley opens the hatch, allowing River and Louisa to escape the deadly trap.

Sean Donovan does not survive the ordeal; he is killed by MI5 agents during the attack. However, his mission is not in vain, as River successfully retrieves the secret file and takes it to his grandfather, David Cartwright, a former intelligence figure. David, concerned about the reputation of MI5 and the intelligence services, burns the file, believing it necessary to protect the institution.

River, however, anticipates this move and had already made a copy of the file, which he keeps hidden in his car. He later leaks the information publicly, exposing Tearney's wrongdoing. This leak leads to the resignations of Ingrid Tearney and the Home Secretary Judd, signaling a significant shake-up in the intelligence community.

At the end of the season, the main Slough House agents--River, Louisa, Marcus, Shirley, and Lamb--are all alive, though Catherine Standish's fate is less clear in the final moments. The team returns to their work, having survived a deadly internal purge and having exposed corruption at the highest levels of MI5.

The season closes on a note of uneasy victory, with River's grandfather's mental decline hinted at, adding a layer of personal conflict to the broader institutional drama. The story emphasizes the resilience of the Slough House team and the high personal costs of espionage within a corrupt and dangerous intelligence environment.

There is no indication of a post-credit scene in Slow Horses Season 3. The available detailed recaps and reviews of the season finale, titled "Footprints," do not mention any post-credit or after-credits scene following the final episode. The finale ends with intense action and significant plot resolutions, but no additional scene after the credits is reported or described in the sources reviewed.

Alison discovered that MI5 boss Ingrid Tierney had approved testing a new device capable of wirelessly hacking computers, which failed disastrously, causing harm to innocent people and nearly killing a North Korean spy. Alison stole a report called Footprint detailing this and intended to leak it, but Tierney ordered her assassination disguised as a suicide.

Diana Taverner orchestrated Donovan's kidnapping of Standish and recommended Webb to be hired by the Tiger Team, indicating her involvement in internal MI5 power plays and betrayals within the service.

River Cartwright's character evolves from someone who is arrogant and self-assured to a more vulnerable state where his confidence begins to unravel, reflecting a necessary humbling process as he faces the season's challenges.

Lamb appears to have insight into the team's activities either through his understanding of their capabilities and personalities or possibly through additional information not explicitly detailed, suggesting his deep experience and connections within MI5.

In the finale, River recovers from a grenade blast while Sean intensifies return fire against Chieftain agents who have breached the facility's lower levels, culminating in high-stakes confrontations that reveal key plot resolutions and character fates.

The TV show Slow Horses, Season 3 (2023), is not family friendly and is generally unsuitable for children or sensitive viewers due to its mature content. It contains severe profanity, including frequent use of strong language and racial slurs, moderate to graphic violence with bloody gunshot wounds and a severed head shown, and mild sexual content such as brief nudity and passionate kissing scenes. There is also moderate depiction of alcohol, smoking, and some intense or frightening scenes.

Specifically, potentially objectionable or upsetting aspects include:

• Frequent and strong profanities, including the f-word used about 20 times per episode and other harsh insults and slurs.
• Graphic violence with multiple bloody shootings, a severed head, and realistic gunshot wounds, especially in the later episodes of the season.
• Mild sexual content including brief male rear nudity, scenes in underwear, and passionate kissing.
• Depictions of alcohol consumption and smoking.
• Intense and frightening scenes that may be disturbing to sensitive viewers.

Because of these elements, Slow Horses Season 3 is intended for mature audiences and is not suitable for children or those sensitive to violence, strong language, or adult themes.