Slow Horses Season 4 Must Finally Answer The Biggest Season 1 Mystery

Mild spoilers for Mick Herron’s Slough House novels that have not yet been adapted by Slow Horses.

Despite its many twists and turns, Slow Horses has yet to resolve a key season 1 mystery. With season 4 of the hit Apple TV+ spy thriller on the horizon, Slow Horses has ample opportunity to delve into the dangling thread — though the source material may give the showrunner pause. Based on Mick Herron’s Slough House series of novels, the series has been praised for its faithfulness to the author’s books. Still, there’s no denying that Slow Horses, which stars Gary Oldman as the slovenly-yet-sharp intelligence officer Jackson Lamb, has taken on a life all its own.

Like the novels, Slow Horses centers on a team of British intelligence agents who have all landed themselves in MI5’s dumping ground department, Slough House. As of 2022, Herron’s Jackson Lamb-starring Slough House series is composed of 13 books — a mix of novels and novellas. Since book 3, The List, is a novella, the first three seasons of the show borrow largely from books one, two, and four — Slow Horses, Dead Horses, and Real Tigers. Slow Horses season 4, however, may want to borrow from future installments in order to resolve a lingering season-one mystery.

Slow Horses Must Answer What Happened To Sid

Slow Horses season 1 introduces viewers to Oldman’s Jackson Lamb, a devious, rude, and incredibly seasoned intelligence officer who heads up Slough House. The first outing also sets up Slow Horses‘ cast of characters in Lamb’s cohort, including series co-lead River Cartwright (Jack Lowden), and a green MI5 agent who landed in the backwater department after making an almost-career-ending mistake during training. Every Slough House agent has a massive misstep that’s brought them to MI5’s dumping ground — except Sidonie “Sid” Baker (Olivia Cooke), an incredibly competent agent who has no business being assigned to Slough House.

In the second episode of Slow Horses season 1, Sid and River tail Robert Hobden (Paul Hilton), a disgraced far-right journalist. The case culminates in a fight that ends with both Hobden and a masked man escaping after the latter shoots Sid in the head. Alive but in critical condition, Sid is taken to the hospital by MI5 security operatives. In episode 5, MI5’s Deputy Director General Diana Taverner (Kristin Scott Thomas) tells Lamb that Sid died in the hospital. Taverner could be bluffing, especially since the finale indicates that all traces of Sid have been wiped from existence.

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What Happens To Sid In The Slough House Books

Several seasons later, it’s still not clear what actually happened to Sid. The revelation in Slow Horse‘s ending that she “doesn’t exist” seems to indicate that Taverner was bluffing about Sid’s death. After all, Sid wasn’t like the other members of Slough House; she had real potential as an MI5 operative, which means Taverner could’ve poached Sid for another role. Not to mention, Jackson Lamb took a liking — as much as he could — to Sid, so Taverner’s reveal about Sid’s supposed death might have been more of a head game meant to mess with Lamb than anything else.

In Herron’s book series, Sid Baker does reappear in the seventh novel, Slough House. Since the Apple TV+ show has been adapting Herron’s stories in order, it seems unlikely that Slow Horses would jump ahead. Still, Sid’s fate is one burning question Slow Horses season 4 shouldn’t shy away from. Regardless of the when, if Sid is alive in the world of Slow Horses as she is in Herron’s books, she’s poised for an eventual return. It would be wise for Slow Horses to wrap up the season-one mystery, all while turning Sid into an even more compelling agent.