Spoiler warning: This article discusses Slow Horses Season 6 through Episode 2.
No, Slow Horses has not confirmed that Sid Baker is working for Frank Harkness. River’s suspicion is reasonable because Sid is alive, hidden and unwilling to explain who protected her, but Episode 2 stops short of connecting her to Harkness or his network.
The unanswered years make Sid look dangerous even when her return may be genuine. River once knew her as a fellow Slow Horse. Now she appears with information and instincts that suggest a much larger secret, while assassins are hunting people linked to Slough House.
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- Release date: September 23, 2026
- Episode: Season 6, Episode 2
- Title: Daddy Issues
- Genre: Espionage thriller
- Streaming on: Apple TV
Sid’s secrecy creates the suspicion
Sid was shot in Season 1 and later disappeared from the official picture. Her sudden reappearance changes River’s understanding of that history. If somebody could keep her survival quiet for years, that person or organization had access, money and a reason to hide her from both MI5 and the Slow Horses.
River cannot simply accept her familiar face as proof of loyalty. Season 6 has already shown that names from Slough House can be obtained and weaponized. Sid may be another intended victim, an inside source, or bait designed to pull River into the open. The episode deliberately preserves all three possibilities.
Harkness is a theory, not the answer yet
Frank Harkness is an obvious suspect because his covert network has already demonstrated its reach and its interest in River. That history gives River a strong reason to test every new development against Harkness. It does not prove that Sid has joined him, willingly or otherwise.
The active murder plot currently points more directly toward Jim and Jane, who are working through a stolen list of current and former Slough House personnel. Their campaign raises the possibility that Sid returned because she knows she is on the list, not because she helped create it.
River needs evidence before he trusts her
Sid’s return is emotionally loaded for River, which makes suspicion a form of self-protection. He wants her survival to be good news, but an intelligence officer who accepts that miracle without checking it would be ignoring the central threat. Her missing history now matters as much as anything she says in the present.
The next test is whether Sid can explain who saved her, where she has been and why she came back now. River also needs to know whether she chose to remain hidden or was prevented from contacting the people who believed she was gone. Until the series answers those questions, Harkness remains one plausible theory rather than a confirmed employer. Do you trust Sid, or is River right to keep his distance? Share your theory in the comments.
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