Slow Horses: who is Sid? River spent three years thinking she was dead — Diana Taverner knew the truth the whole time
Sidonie "Sid" Baker is the Slough House agent who was never supposed to be there. In season 6, she turns up alive in River Cartwright's dead grandfather's house, and the explanation leads straight back to Regent's Park.
Olivia Cooke played Sid in the first season of Slow Horses in 2022 and then vanished from the show. She returned at the end of season 6, episode 2, "Daddy Issues," which streamed on Apple TV on September 23, 2026. The six-episode season premiered on September 16, runs weekly through its finale on October 21 and adapts two of Mick Herron's novels, Joe Country and Slough House.
Spoilers for Slow Horses through season 6, episode 4 follow.
Who Sid was in season 1
Sid stood out at Slough House because she was competent. Everyone else there had been exiled from MI5 headquarters after a career-ending mistake. Sid had no visible disgrace, and before the season was over she told River why: First Desk Diana Taverner (Kristin Scott Thomas) had placed her there to keep an eye on him.
Shortly afterward, she was shot in the head. She and River had followed a masked intruder into journalist Robert Hobden's house, and the intruder, former Slow Horse Jed Moody, fired as he ran. What followed looked like a death written out in installments:
- The hospital — River held Sid until the ambulance came. She made it to intensive care, and Jackson Lamb told River the doctors thought she might live.
- The visit — River saw her once more, unconscious in her hospital bed, in episode 4.
- The word — Lamb later told River she was gone.
- The loose end — in the season 1 finale, Roddy Ho found no record that Sid had ever been treated at the hospital, and no record of Sidonie Baker in MI5's database at all.
Roddy pointed out at the time that she might still be alive somewhere. The show then went five seasons barely mentioning her.
How she comes back
Season 6 opens with someone killing former Slow Horses. Struan Loy and Kay White are murdered in the premiere, and River's grandfather, the MI5 legend David Cartwright, has died offscreen. River starts getting silent phone calls, assumes they're from his father, Frank Harkness (Hugo Weaving), and has Roddy trace them. They're coming from his grandfather's empty house. When he gets there, Sid is waiting.
In episode 3, "Resurrection," River admits he has spent three years believing she was dead. Sid isn't in the mood to comfort him.
"So I got shot in the head, but you're the one that's traumatized?" she asks him.
The same pair of assassins hunting the other Slow Horses, played by Kyle Soller and MyAnna Buring, are there for her too. Sid explains that a few weeks earlier, travel paperwork connected to Kazan, Russia, was sent to her home. The Park dismissed it as a glitch. Then strangers started asking about Sid Baker. River and Sid find a reported killing in Kazan, a woman shot twice in the head and written off as a robbery, and the shape of it becomes clear. Someone used the identity of an officially dead MI5 officer for an off-the-books operation in Russia, and the real Sid has become a loose end.
What Diana Taverner knew
Sid's instinct is to call Taverner for help. River tells her not to, on the logic that if the Park told everyone Sid was dead, Taverner may know exactly why a dead woman's identity was useful.
Episode 4, "Lost and Found," which streamed on October 7, proves him right in the worst way. JK Coe is killed, Sid is abducted, and the abduction turns out to be part of a deal Taverner has struck with the Russians: she surrenders Sid in exchange for an end to the killings of the other agents.
Cooke has described what Sid's missing years looked like. In an interview with The AU Review, she said the character went through a secret rehabilitation facility for injured field agents and was then kept at a secure location in Cumbria, told to wait until she was needed.
Cooke's summary of those years was blunt: "She's basically been a prisoner." She has also said Taverner told Sid almost nothing about her own situation.
What the books say
Readers knew this was coming. In Herron's Slough House, the seventh novel, published in 2021, River finds Sid sitting in his grandfather's study, years after she was declared dead. She had been treated in secret and resettled under a new identity in Cumbria. The Kazan killing is Taverner's unauthorized retaliation for a Russian nerve-agent attack in Britain, paid for with private money, and the Russian response is aimed at Slough House.
Book spoilers beyond this point. The novel ends with River poisoned by a nerve agent smeared on his grandfather's door and Sid holding him, begging him to stay alive. By Clown Town, the ninth book, River is out of a coma and Sid is his partner. Apple renewed the series for an eighth season on September 15, 2026, and that season will adapt Clown Town.
Two episodes of season 6 remain. River is chasing the car that took Sid, and the woman who traded her away still runs MI5.
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