Updated

Updated · Variety · Oct 21

Olivia Cooke Returns to Slow Horses After 4 Seasons as Season 6 Ends on Poisoning Cliffhanger

Updated

Updated · Variety · Oct 21

Olivia Cooke Returns to Slow Horses After 4 Seasons as Season 6 Ends on Poisoning Cliffhanger

Q: After Sid's shocking return and River's sudden poisoning, who is the real traitor in the Slow Horses Season 6 finale?

The apparent traitor is Diana Taverner, but the finale’s immediate poisoning twist points more directly to the Russian operatives’ trap than to a fresh betrayal by Sid. Season 6 makes clear that Taverner’s off-the-books Russia retaliation operation set the whole crisis in motion. She used Sid’s identity as cover, exposed Slough House, and was prepared to hand Sid over to the GRU. That is the deeper act of betrayal driving the finale, and it is why Sid finally realizes she was expendable. But River’s collapse at David Cartwright’s house is not presented as proof that Sid betrayed him. Reporting tied to the episode and to Mick Herron’s books indicates River was likely poisoned by Novichok after touching a contaminated object in the house — in the show, apparently the crowbar, planted earlier by the Russian assassins Jim and Jane. In the novels, a similar poisoning happens via the house itself, and River survives. So the “real traitor” in political and institutional terms is Taverner, whose decisions sacrificed her own people. In the final scene’s practical terms, the attack on River appears to be the work of the Russians’ poison trap, not a double-cross by Sid. The finale is designed to leave that tension hanging, but the available evidence points away from Sid and back toward the consequences of Taverner’s operation.

Q: Will Olivia Cooke's traumatized MI5 agent finally get her revenge on Diana Taverner after surviving a brutal four-season absence?

Not yet. Season 6 positions Sid Baker for a reckoning with Diana Taverner, but it does not deliver that revenge on screen. The finale makes clear why Sid would want it. After surviving the Season 1 gunshot, she spent years hidden away and recovering while MI5 let the world think she was dead. In Season 6, she learns that Taverner used her identity as cover for an off-the-books killing in Russia, effectively making her expendable and putting former Slough House agents in danger. Cooke has said Sid’s emotional turn comes when she realizes Diana may not have saved her at all, but sacrificed her. That realization leaves Sid “ready to fight for herself.” There is strong evidence the show is building toward a Sid-versus-Taverner confrontation. Cooke herself said she would love that showdown, and the season’s story gives Sid a clear personal motive. The source novels also support continuing fallout from Taverner’s decisions, though TV adaptations do not always follow the books beat for beat. For now, the bigger immediate question is survival, not revenge. The finale ends with River collapsing after apparently being poisoned by residue left on the crowbar he handled, while Sid is upstairs and unaware. That cliffhanger strongly suggests any revenge plot has been deferred to a future season. So the answer is that Sid now has both motive and momentum, but whether she finally gets revenge on Taverner remains unresolved. The series has set it up; it has not paid it off yet.

3 articles · Updated · Variety · Oct 21

Four seasons after Sid Baker was presumed dead, Olivia Cooke said rejoining “Slow Horses” put her back at the center of Season 6’s finale, which ends with River Cartwright collapsing from an apparent poisoning.
Season 6 sends Sid and River on the run from Russian sleeper agents after MI5 chief Diana Taverner used Sid’s identity to cover an off-the-books assassination on Russian soil, making Sid and Slough House targets for revenge.
Five weeks of mostly night shoots drove Cooke’s return, and she said the toughest injury came not from explosions or truck stunts but from pulling both quad muscles while running during a 2 a.m. chase.
Cooke said Sid returns more hardened after years of hidden rehabilitation and finally sees Taverner’s ruthlessness clearly, while her unresolved bond with River becomes the emotional engine of the finale.
The comeback also lands as “Slow Horses” has grown into an Emmy-recognized hit and Cooke’s profile has risen through “House of the Dragon,” setting up a bigger role if Sid returns again.