First, the good news: Slow Horses has officially been renewed for an eighth season. What began in 2022 as a relatively low-profile British spy series has become one of Apple TV’s strongest titles, collecting awards, nominations, and an increasingly passionate fan base along the way. And Season 6 has not even started yet.

The new season, premiering September 16, will combine Joe Country and Slough House, the sixth and seventh books in Mick Herron’s series. Season 7 will adapt Bad Actors. Season 8 will move on to Clown Town, the ninth and most recent novel in the main series. Which means Apple is about to run out of books.

For anyone who watched Game of Thrones move beyond George R.R. Martin’s published material, that alone is enough to raise an uncomfortable question. Knowing where a story may eventually end is not the same as having the books showing you how to get there. Could Slow Horses face the same risk?

Perhaps, but there are important differences. Herron is still writing and has already suggested that he knows the long-term fate of his characters. For now, at least, there is no sense that the television series has been left without its creator’s guidance.

The adaptation has also always enjoyed more freedom than Game of Thrones. Slow Horses has changed characters, rearranged events, and will now condense two novels into a single six-episode season. Its fidelity has never been entirely literal, but tonal: the cynicism, the British humor, the bureaucratic absurdity of MI5 and Jackson Lamb’s peculiar moral code. Perhaps that is exactly what will protect it.

Season 8 will put Lamb and Diana Taverner through what Apple describes as their most explosive confrontation yet, after a botched attempt to blackmail the Park exposes one of MI5’s darkest secrets. Frankly, the promise of Gary Oldman and Kristin Scott Thomas trying to politely destroy each other is already reason enough to celebrate the renewal.

What is most striking is how much the series has grown. Jackson Lamb first appeared as an apparently repellent figure, while River Cartwright seemed destined to occupy the traditional role of the young hero. Instead, Slow Horses has become more eccentric, more cynical and more ensemble-driven with every season. Slough House, created as a dumping ground for MI5’s failures, somehow became exactly the place audiences never want to leave. And that is why the next question matters.

By the time Season 8 reaches Clown Town, Apple will have adapted all nine main novels published so far. There are novellas and other stories within Herron’s universe, but at some point the series will have to choose between waiting for new material, exploring side stories, or moving beyond the books. And that choice can change a television series.

For now, though, there are far more reasons to celebrate than to worry. Mick Herron is still writing, Gary Oldman is still Lamb, and Slow Horses has shown no sign of losing momentum. But for the first time, the future of the series may depend not only on Apple, but also on what Herron writes next.

The horses may be slow. Television has finally caught up with them.