Spoilers are ahead for Slow Horses season 3.

Summary

• Slow Horses season 4 will introduce a new character, Claude Whelan, who will have a significant impact on the story.
• Season 3's finale, which implies Diana Taverner will claim First Desk, deviates from the source material.
• Whelan's potential promotion to First Desk instead of Taverner in season 4 will only deepen the internal strife within MI5.

Based on the Slough House novels by Mick Herron, Apple TV+'s Slow Horses deviates from its source material at times. However, one crucial change has become more apparent in the wake of Slow Horses season 3, which adapted Herron's Real Tigers. By and large, the spy thriller series has been faithful to its source material. As in the novels, the rude-yet-incisive veteran MI5 agent Jackson Lamb (Gary Oldman) heads up Slough House, a purgatory for MI5 agents who've made career-ending missteps. Although these "slow horses" are relegated to drudgery, they get involved in high-profile cases.

Aside from impertinent intelligence officer Jackson Lamb, Slow Horses' other key characters include River Cartwright (Jack Lowden), a newly minted MI5 agent who lands in Slough House after making a very public mistake, and Diana Taverner (Kristin Scott Thomas), the Deputy Director General of MI5 and head of operations. Unsurprisingly, Lamb has a complicated relationship with Taverner, who's MI5's designated "Second Desk." In Herron's novels, there are five desk positions that oversee various MI5 departments, though all of them report to the coveted First Desk — a position Taverner has been eyeing, much to Slough House's dismay.

Ahead of season 4's release, Slow Horses has already been renewed for a fifth outing.

Whelan Will Have A Huge Effect On Slow Horses Season 4

In Slow Horses seasons 1 through 3, Ingrid Tearney (Sophie Okonedo) serves as the Director General of MI5 — a.k.a. the "First Desk." While Taverner learns to trust in Lamb's crew, Tearney thinks very little of the disgraced slow horses. In order to test MI5's security efforts, Tearney sets up the kidnapping of an office , Catherine Standish (Saskia Reeves). When the abduction spirals out of MI5's control and becomes a real threat, Tearney is forced to turn to Slough House for help. Standish's captor, Sean Donovan (Sope Dirisu), reveals that he wants a confidential file, "Footprints."

Donovan plans to leak the contents of Project Footprints in order to expose MI5 corruption at the highest level. At the end of Slow Horses season 3, it's implied that River leaks the file and ends Tearney's reign. "It’s you in the hot seat now, Diana," Lamb tells Taverner, who's taking on First Desk duties. However, Taverner's promotion deviates from Herron's novels. In Slough House's fourth book, Spook Street, Claude Whelan replaces the disgraced Ingrid Tearney as Director General of MI5. Interestingly, Slow Horses has cast James Callis as Whelan, which has massive implications for Taverner's season 4 story.

What Happens To Whelan In The Books

Jackson Lamb's final conversation with Taverner in Slow Horses season 3's finale takes place before Taverner heads to a meeting. Given everything that's unfolded internally for MI5, and given that she's primed for the role, Taverner assumes that she'll officially be made First Desk in her post-stroll-with-Lamb meeting. Viewers don't get to see how that conversation pans out, leaving the season-four holder of the First Desk somewhat up in the air. Since Whelan wasn't featured ahead of Slow Horses season 4, it's possible Whelan's introduction in the fourth outing will see him usurping Taverner's spot.

According to the series' showrunners, Slow Horses season 4 should be released by the end of December 2024.

In the books, River isn't the one who leaked the Project Footprint files. Instead, the information lands in Jackson Lamb's hands. Unlike River, Lamb refuses to hand over the files, which ends up impacting Taverner's campaign for First Desk. Spook Street sees Taverner continuing her Second Desk role and heading up MI5's Operations. By the end of the fifth book in Herron's series, London Rules, the pressures of First Desk catch up with Whelan. After Whelan is ousted from the position, Taverner finally ascends to her long-coveted role in the sixth full-length Slough House novel, Joe Country.

Slow Horses Season 4 Needs Whelan

Since Callis has been cast as Whelan, it seems likely that Slow Horses season 4 will use the character to thwart Diana Taverner's ascendancy to First Desk. Not only would Whelan's unexpected promotion to First Desk in season 4's opening keep the series more in line with the books, but it would provide Taverner with yet another obstacle to overcome. MI5's Second Desk holder has already proven to be devious, especially when it comes to her personal ambitions. Plus, depicting more internal strife within MI5 keeps the stakes high, especially when so much of Slow Horses boils down to trust.