Slow Horses season 4 recap: Everything to know before season 5 arrives

Apple TV+’s hit spy drama Slow Horses season 4 concluded with a mix of shocking revelations, emotional goodbyes, and unfinished business that paves the way for the next installment. With Slow Horses season 5 set to premiere on September 24, fans are eager to revisit the chaos that unfolded in Slow Horses season 4.

From River Cartwright’s devastating family discovery to the heartbreaking death of a beloved team member, the season raised the stakes for Slough House in ways that will have lasting consequences.

What happened in Slow Horses season 4?

Slow Horses season 4 begins with a devastating bombing at a shopping center in London. The incident quickly draws in MI5 and the agents of Slough House, who uncover that the mastermind behind the attack is Frank Harkness, played by Hugo Weaving. Harkness is revealed as a former intelligence operative with a chilling plan: to build a mercenary army made up of his sons.

The personal stakes rise when River Cartwright learns that Harkness is his biological father. This revelation creates one of the most intense confrontations of the season, forcing River to grapple with both his family history and his loyalty to Slough House. Despite River’s efforts to bring Harkness down, the finale ends with Harkness walking free, thanks to the leverage he holds over MI5. His release guarantees that the threat he poses will carry over into season 5.

Tragedy also strikes the team when Marcus Longridge is killed during a shootout with Harkness’s men. Known for his easygoing nature and camaraderie, Marcus’s death deeply impacts his colleagues, especially Shirley Dander. Their bond had been one of the season’s most heartfelt dynamics, and Shirley’s grief, compounded by her ongoing struggles, sets up a vulnerable arc for her moving forward.

The finale further underscores the instability within Slough House. Catherine Standish remains absent after resigning, while Jackson Lamb continues to hold the group together with his unconventional leadership. Meanwhile, River faces another personal blow when he is forced to place his grandfather David Cartwright in assisted living due to his worsening memory.

By the time Slow Horses season 4 closes, the team is left fractured and uncertain. Harkness remains at large, Diana Taverner is still navigating MI5 politics, and River is caught between duty and personal loss.

Frank Harkness as the central villain

One of the standout elements of Slow Horses season 4 is Frank Harkness as the ultimate antagonist. His ruthless ambition, combined with his willingness to sacrifice his own children for power, cements him as one of the most dangerous foes Slough House has ever faced. His twisted bond with River not only complicates the mission but also injects deep emotional tension into the story.

Harkness’s unpredictability makes him even more menacing. At times, he spares lives when least expected, only to strike with brutal precision later. His release in the finale ensures that his shadow looms large over the events of the next season.

The impact of Marcus’s death

The death of Marcus Longridge in Slow Horses season 4 marks a turning point for the series. His loss disrupts the fragile sense of unity within Slough House and leaves his teammates emotionally shaken. Shirley, who had a close bond with him, is left reeling and questioning her own choices. The event also highlights the dangers the team faces in every mission, reminding audiences that no one is safe in this world of espionage.

Jackson Lamb’s response to Marcus’s death further underscores his complicated role as both a leader and a reluctant father figure. Beneath his abrasive demeanor, Lamb shows glimpses of the loyalty and care that bind the group together, even as they mourn.

Lingering questions heading into season 5

The unresolved threads of Slow Horses season 4 leave plenty of anticipation for the next installment. Harkness’s survival and release raise urgent questions: Will he rebuild his mercenary operation? Will River have to face him again, this time with even higher stakes?

Diana Taverner’s ongoing struggles within MI5 also promise more intrigue. Her clashes with Claude Whelan highlight her ambition and her determination to outmaneuver rivals. Whether she can finally rise to the top remains to be seen.

River’s personal life adds another layer of complexity. His painful decision regarding his grandfather and his fraught relationship with Harkness give his character new depth. How he balances these burdens while continuing to serve Slough House will be central to season 5.

Slow Horses season 5 will premiere on Wednesday, September 24, 2025.

About the author

With over five years of work experience, Bea Melisse Ibañez is a seasoned writer covering K-pop, K-dramas, and the latest trends in the entertainment industry at Sportskeeda. Holding a Bachelor of Arts degree in Journalism, Bea combines her academic background with her fascination for Hallyu culture, creating insightful and accurate content.

In her career, Bea has had the opportunity to interview prominent figures in the K-pop industry, including groups like TWICE and artists such as GOT7's BamBam, Lee Seung-gi, and Hwang Min-hyun.

As a pop-culture enthusiast, her love for Korean dramas and music began in high school, where she discovered 2nd and 3rd-generation K-pop groups. She also admires actors like Eddie Redmayne, Lee Joon-gi, and Park Eun-bin, and her favorite celebrity is Miley Cyrus.

In her free time, Bea likes to rewatch her favorite shows. If given a chance, she would like to be a part of the dazzling world of Hazbin Hotel.

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Edited by Bea Melisse Ibañez