‘Slow Horses’ Season 6 Widens Slough House’s World With a New Wave of Guest Stars

Apple TV+ has brought ‘Slow Horses’ back for a sixth run, and this time the misfit spies of Slough House are not chasing a mission so much as running from one. The season has arrived with an expanded guest roster that sits alongside the show’s familiar ensemble, and the mix of new faces gives the spy thriller a noticeably different shape.

The season follows Jackson Lamb and his team as a hit list tied to Slough House sets off a wave of killings, forcing the agents into hiding rather than into the field. Alongside that central plot, a set of new performers have joined ‘Slow Horses’ to play everyone from a pair of hired killers to a media mogul with his own agenda.

Slow Horses Season 6 Guest Stars Bring New Threats to Slough House

The season’s most prominent new additions are Kyle Soller and MyAnna Buring, who play a pair of assassins hunting people connected to Slough House. The two are introduced as an assassin couple hunting down the Slough House agents who are being positioned as the season’s primary antagonists. Their characters attack surviving agents as part of a larger, more sinister plot that may reach back to Diana Taverner.

Reviewers have singled out the pairing as an effective addition to the show’s tone. Soller, known for ‘Andor’, and Buring, known for ‘The Witcher’, have been described as two of the most menacing villains the series has produced, dispatching the Slow Horses with unsettling ease. Their arrival changes the rhythm of the season, since the team is no longer stumbling into danger but is instead actively being hunted.

Harry Lloyd also joins the cast this season as Damien Cantor. Lloyd, known for ‘The Fear’, plays a media mogul and news channel owner in the new season. His character is a media figure who manages to knock Taverner down a peg, and the role has been noted as one that fits smoothly into the show’s established world of political maneuvering.

Lucian Msamati rounds out the new guest players as Oliver Nash. Msamati, recognizable from ‘Game of Thrones’, takes on the role of Oliver Nash this season. Rotten Tomatoes lists Msamati among the season’s credited guest stars, alongside Joanna Scanlan’s Moira Tregorian.

New Cast Additions Include a Young Witness at the Center of the Season

Lenny Rush joins ‘Slow Horses’ this season as Reece Nesmith, a character who becomes tied to the investigation Lamb is running. Rush, known for ‘Am I Being Unreasonable?’, plays Reece Nesmith, described as a child witness. Advance reviews identify Reece as a young witness who ends up interacting directly with Lamb and Roddy Ho, and Rush previously won a BAFTA TV Award for ‘Am I Being Unreasonable?’ while also appearing in ‘Doctor Who’.

His character’s arc ties into the season’s central threat, since Frank Harkness resurfaces to target the people connected to the ongoing killings. River’s mercenary father Frank Harkness, played by Hugo Weaving, resurfaces to throw a wrench in the plot by targeting Lamb’s key witness Reece Nesmith. The character gives the season a plot thread that runs somewhat separate from the main Slough House team.

Naomi Wirthner and Paul Higgins also appear this season in supporting roles tied to the wider MI5 world. Naomi Wirthner plays Molly Doran, while Paul Higgins returns as Struan Loy, another original Slow Horse who had not been part of the regular Slough House crew since the first season. Loy’s fate is central to the season’s opening, as the character meets his killers early in the premiere before the wider hunt on the team begins.

Returning Slough House Cast Members Take on Bigger Roles

Several familiar performers have expanded parts in Season 6 rather than simply reprising their earlier roles. Olivia Cooke, Ruth Bradley, Samuel West, and Joanna Scanlan return this season with elevated involvement compared to their previous appearances. Cooke’s return as Sid Baker has been called one of the biggest casting surprises of the season, since the character had been absent from the show since its first season.

Ruth Bradley returns once again as Emma Flyte, the head of MI5’s internal security unit. Speaking with Town & Country ahead of the season, Bradley described returning for another season as feeling like going home and called the experience absolutely joyous. She also noted that her character had entered the previous season with more humility and awareness after an earlier setback in the role.

Samuel West continues his recurring turn as Peter Judd, a politician whose scheming has run through several seasons of the show. West’s character returns for Season 6 with renewed confidence and an expanded part in the story. Joanna Scanlan also returns as Moira Tregorian, an administrator whose ties to Slough House give her a bigger presence in the season’s events.

The Core Slough House Team Anchors Season 6 Amid the New Faces

Gary Oldman remains the central figure of ‘Slow Horses’ as Jackson Lamb, the head of Slough House. Lamb spends the season investigating the deaths of former Slough House agents, gradually realizing that someone is systematically targeting both past and present members of the team. That investigation puts him at odds with Kristin Scott Thomas’s Diana Taverner, whose priorities do not always align with his.

Jack Lowden also returns as River Cartwright, whose season is shaped by loss and by his father’s reappearance. River is dealing with the death of his grandfather David while also becoming a target of the assassins, and Frank Harkness’s return gives him a significant personal storyline. Saskia Reeves, Christopher Chung, Rosalind Eleazar, Aimee-Ffion Edwards, and Tom Brooke round out the returning core cast as Catherine Standish, Roddy Ho, Louisa Guy, Shirley Dander, and J.K. Coe.

The season also marks a structural shift for the show’s source material. The six episode season begins September 16, 2026, and adapts two of Mick Herron’s novels, ‘Joe Country’ and ‘Slough House’. This makes Season 6 the first to combine two mainline novels into a single season, whereas previous seasons largely followed one book apiece. Adam Randall directed all six episodes of the season.

With this many new arrivals layered onto an already dense ensemble, Season 6 asks longtime viewers to track more moving pieces than usual, and it will be worth hearing which of these new additions, from the assassins hunting the team to Reece Nesmith’s role in Lamb’s investigation, left the strongest impression on fans watching the season unfold.