The 5 biggest unanswered questions from Slow Horses Season 4

Season 4 of Slow Horses ended in typical Slough House fashion: concise plotlines, dry wit, and just enough resolution to move things forward, while leaving a few loose ends dangling. As the overarching narrative involving Frank Harkness reached its end, not every character was left intact, and several subplots remain frustratingly unresolved. The show's fans know better than to search for tidy resolutions, and this season doubled down on that mantra.

But amid the blackmail, bombings, and ultimate rescues, the season did more than just close a door on one villain. It opened the scope, quietly shifting its attention to wider cracks in British intelligence and the dubious fate of its lesser-known agents. So, what wasn’t answered?

Below are the 5 biggest unresolved mysteries Slow Horses Season 4 left behind

1. What's River Cartwright going to do next in Slow Horses?

River Cartwright’s Season 4 was intensely personal. From battling Frank Harkness, revealed to be his father, to placing his grandfather David Cartwright into assisted living, River ended the season alive but physically exhausted. His status is safe, but his next move is not.

Does he remain at Slough House under Lamb? Or is he heading for a departure from MI5 entirely? The episode concludes with him sipping a drink in companionable silence with Lamb, but leaves his future career direction uncertain.

2. What is Diana Taverner's long-term game in Slow Horses?

Diana Taverner weathered another political and operational storm. Blackmailed into freeing Harkness, she’s battered but still powerful. Her reputation is tarnished, but she remains Deputy Director-General, suggesting her influence is far from over.

The question is how she will maintain her grip on MI5’s future. Will she seek revenge, reshape the agency, or focus on damage control? Her choices in the coming months could reorder the intelligence pecking order.

3. What did the final call to Jackson Lamb in Slow Horses really mean?

Jackson Lamb receives a mysterious phone call toward the end of the season, one that arrives and ends abruptly but weighs heavily. Nothing is said, but it’s clear that something—or someone—is coming. Lamb, being Lamb, doesn’t register much emotion, but the show clearly intends the phone call to be a turning point for what will unfold next.

Was it just a heads-up or the start of something much worse? The ambiguity makes it one of Season 4’s most frustrating loose ends.

4. How does Louisa handle Marcus's death in Slow Horses?

Marcus Longridge makes a heroic sacrifice during the season finale, being shot while protecting David Cartwright. Louisa Guy emerges unharmed from the ordeal, but the psychological impact of Marcus’s death is glossed over—perhaps intentionally.

Their dynamic had been built throughout the show, and with Marcus’s death, Louisa’s role in Slough House will be altered. Will she become closer to the team or more withdrawn? The show doesn’t provide a clear path, leaving fans to wonder how this loss will impact her moving forward.

5. What will Frank Harkness' future role be in Slow Horses?

Frank Harkness is technically defeated at the end of Season 4—his plans are exposed, his familial connections revealed—but he escapes legal consequences through blackmail. One wonders if he might reappear in later seasons, possibly from a different angle, given this loose thread.

Is this the start of something greater, or will he continue to lurk in the shadows? It’s hard to believe we’ve seen the last of him, considering the chaos he has already caused.

Season 4 of Slow Horses quietly introduces new plotlines while closing others. The situation with MI5 and Slough House is far from settled, even though River survives and Frank is currently out of the way. The aforementioned questions are intentional narrative gaps rather than the result of incomplete storytelling.