The Shocking Truth: Who Is Frank Harkness? A Deep Dive Into The Slow Horses Season 4 Cast And Their Biggest Twist

The dysfunctional, yet utterly compelling, world of Slough House is back in the spotlight, and as of December 10, 2025, the cast of Slow Horses remains one of the most prestigious ensembles on television. This Apple TV+ spy thriller series, based on the celebrated novels by Mick Herron, follows a group of disgraced MI5 agents—known as "Slow Horses"—who have been exiled to the administrative purgatory of Slough House under the watchful, slovenly eye of Jackson Lamb. The show's success hinges on the perfect balance between high-stakes espionage and the darkly comic incompetence of its protagonists, a feat only possible with a world-class cast.

The latest season, Season 4, delivered the most explosive twist yet, introducing a new villain whose identity sent shockwaves through the fandom and directly impacted one of the main agents. This deep-dive article breaks down the core cast, the new additions, and the pivotal character reveal that defines the current state of the "Slow Horses" universe.

The Slough House Roster: Full Cast and Character Profiles

The heart of Slow Horses lies in the complex relationships between the agents, from the perpetually annoyed Jackson Lamb to the ambitious River Cartwright. Here is a look at the main actors and the key entities they portray, including their recent career highlights.

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Gary Oldman as Jackson Lamb

• Character Profile: The brilliant but repulsive head of Slough House. A Cold War veteran and former MI5 star agent, Lamb is now a cynical, flatulent, and often drunk spymaster who secretly retains his sharp instincts.
• Biography & Recent Work: Leonard Gary Oldman is an Academy Award-winning English actor known for his chameleonic roles. Prior to Slow Horses, he was famous for playing Commissioner Gordon in The Dark Knight trilogy and Sirius Black in the Harry Potter films. His portrayal of Jackson Lamb has garnered him significant critical acclaim, including a BAFTA nomination.

Jack Lowden as River Cartwright

• Character Profile: The grandson of a legendary MI5 figure, David Cartwright, River is a talented but reckless agent desperate to return to Regent's Park (MI5 headquarters). His demotion to Slough House was the result of a botched training exercise.
• Biography & Recent Work: Jack Lowden is a Scottish actor born in 1990. He gained recognition for his roles in major films like Dunkirk (2017) and Mary Queen of Scots (2018). His upcoming projects include the 2025 film Tornado, demonstrating his continued rise in the industry.

Kristin Scott Thomas as Diana Taverner

• Character Profile: The "Second Desk" at MI5, Taverner is a ruthless, high-ranking intelligence officer who often uses the Slough House agents for her own clandestine operations. She is Lamb's nemesis and former colleague.
• Biography & Recent Work: Dame Kristin Ann Scott Thomas is a celebrated Anglo-French actress. An Oscar nominee for The English Patient, she is known for her poise and elegant work across film and theatre. She continues to be a defining figure in British and French cinema.

The Core Slough House Team

• Saskia Reeves as Catherine Standish: Lamb's long-suffering administrative assistant, a recovering alcoholic with a deep, complicated history with both Lamb and the late MI5 chief.
• Rosalind Eleazar as Louisa Guy: A sharp, pragmatic agent whose relationship with her colleague Marcus Longridge is a key dynamic.
• Christopher Chung as Roddy Ho: The brilliant, arrogant, and socially inept computer expert who is essential to the team's operations.
• Aimee-Ffion Edwards as Shirley Dander: A newer, aggressive agent who brings a different kind of energy to the Slough House office.
• Kadiff Kirwan as Marcus Longridge: A former MI5 agent with a gambling problem, now a reliable field operative.

The Season 4 Twist: Hugo Weaving as Frank Harkness

The fourth season of Slow Horses, which premiered in late 2024, introduced a new level of personal stakes for the Slough House team, particularly River Cartwright. The central mystery of the season, involving a London bombing and a deadly incident involving River's grandfather, David Cartwright (played by Jonathan Pryce), led directly to the introduction of a major new villain.

Hugo Weaving as Frank Harkness: River's Estranged Father

The most significant new addition to the cast is screen legend Hugo Weaving, who takes on the role of Frank Harkness. Weaving is famous for iconic roles like Agent Smith in The Matrix and Elrond in The Lord of the Rings. His casting alone signaled a major threat, but the character's reveal was the true shock.

Harkness is introduced as a rogue intelligence operative and a "ruthless" villain whose plan is central to the season's conflict. The finale delivered the jaw-dropping confirmation: Frank Harkness is River Cartwright's estranged father. This revelation not only explains River's deep-seated issues and his grandfather's paranoia but also positions Harkness as the most personal antagonist the young agent has ever faced.

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The dynamic between Weaving's cold, calculating Harkness and Lowden's frustrated, determined River Cartwright provides the emotional core for the season, elevating the spy drama beyond typical procedural work. This new family dynamic is set to have lasting repercussions on the future of Slough House and the entire series.

Other Key New Entities and Season 4 Plot Details

Season 4, based on Mick Herron's novel Spook Street, focuses on the fallout from a terrorist bombing and the subsequent investigation that pulls the Slow Horses into a high-profile web of secrets.

Joanna Scanlan as Moira Tregorian

Another notable new entity is Moira Tregorian, played by the highly accomplished actress Joanna Scanlan. Scanlan's character is a new addition to Slough House, though she is initially confused about her placement among the disgraced agents. Scanlan, known for her BAFTA-nominated work, brings her considerable talent to the ensemble, further cementing the show's reputation for attracting top-tier British acting talent.

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The David Cartwright Incident

The season's inciting incident involves River’s grandfather, David Cartwright (Jonathan Pryce), a retired MI5 legend who kills a home intruder. It is revealed the intruder was an imposter planning an assassination. This event, driven by David’s increasing dementia and paranoia, is what first draws River and the team into the dangerous new operation, ultimately leading them to Frank Harkness.

Looking Ahead: The Future of the Slow Horses Cast

With Season 4 having concluded, the confirmed Season 5, based on the novel London Rules, promises to continue the high-stakes drama. The cast, led by the incomparable Gary Oldman, is expected to return in full, with the lingering threat of Frank Harkness and the political machinations of Diana Taverner continuing to loom large. The introduction of major new entities like Hugo Weaving and Joanna Scanlan ensures the show maintains its fresh, high-quality standard, keeping the audience hooked on the misadventures of the world's most entertaining group of failed spies.

The show remains a critical darling, praised for its sharp writing, dark humor, and, most importantly, the stellar chemistry of its ensemble cast. The latest twists have not only provided immediate dramatic payoff but have also set the stage for a compelling future for the Slough House agents.