So much happens at once in the fourth episode of Slow Horses’ sixth season, and I finished it with a heavy heart. It is an excellent episode, the kind that barely lets us catch our breath between threats, but it also makes the challenge of bringing two books together in a single season more apparent. We follow two stories that run in parallel, and part of the tension comes from trying to understand where they intersect. Frank Harkness, the Russian assassins, Diana’s decisions, and Sid’s return involve the same characters, but the interests behind each move still need untangling.

Lamb makes progress by using Reece to provoke a response from whoever hired his father to steal the Park’s files. The demand for money reaches Damien Cantor, who calls in Frank, and the meeting at Shake Shack brings together two men who know exactly how this world works. Lamb wants to discover whether Harkness is behind the attacks on his agents, while Frank tries to control the conversation and assess what his adversary knows. The presence of Harkness’s men and their discovery of Reece nearby make the situation even more dangerous, until Lamb turns a public disturbance into an opportunity to escape with the boy.

For me, this scene between Lamb and Harkness is the best thing in the season. Gary Oldman and Hugo Weaving give the encounter a tension that needs no physical confrontation to work. The pleasure lies in the dialogue, the taunts and the sense that each man is trying to find the other’s vulnerable spot. When Frank mentions having tracked Lamb down in Berlin, where he was using another name, the conversation takes on a particularly disturbing dimension. After the revelation of Lamb’s scars at the end of season five, we know that this past holds a deeply personal trauma. The reference deserves a discussion of its own, but it already gives the encounter the unsettling suggestion that Harkness knows more than Lamb would like.

While Lamb tries to establish Frank’s role, the operation in Kazan helps explain why the Slow Horses are being hunted. A woman connected to Russian intelligence was assassinated, and the GRU wants to respond to the action attributed to the British. The problem is that someone used Sid’s identity, making her responsible for a mission she did not take part in. The list of Slough House agents broadens that revenge: the Russians believe they have identified a British assassination unit and begin hunting its members, including those who have already left the department. The Slow Horses end up paying for an operation they knew nothing about.

Diana tries to distance herself from responsibility for Kazan, but she is increasingly cornered by Oliver Nash’s questions, Cantor’s pressure and Vassily’s demand. When the Russian offers to stop the attacks in exchange for Sid, she agrees to hand the agent over. It is a brutal negotiation, taking place while Sid still believes Diana will protect her. The gap between what one expects and what the other is willing to do makes Sid’s attempts to contact her particularly agonizing: she keeps trying to reach the very person who has already agreed to sacrifice her.

Meanwhile, Standish, Shirley, Louisa and Moira need somewhere to hide, and Lamb solves the problem with a suitably provocative move: he steals the keys to Diana’s country house and sends his nags there. It is one of the episode’s best ironies, because the MI5 chief’s property becomes a refuge for the agents paying for the consequences of an operation connected to her. With so much happening elsewhere, this strand keeps the group together and preserves the show’s humor: even choosing a hiding place allows Lamb to antagonize Diana while trying to keep his people alive.

Jim and Jane’s pursuit keeps River, Sid and Coe on the move, and the bowling alley sequence combines humor and danger beautifully. River and Sid need a phone, but getting access to it means overcoming an employee’s resistance as he closes for the night. Sid solves the problem by inventing a family emergency, demonstrating the ease with which she lies that so irritates River. Their chemistry continues to work, even when their time to talk is constantly interrupted by the need to escape.

When the assassins catch up with them, the situation deteriorates again. River injures his hand, and the trio eventually seeks shelter in a shopping center, where the presence of other people seems to offer some protection. The respite is precarious, because Jim and Jane arrive there too. Sid remains determined to speak to Diana and uses Coe’s phone, but the response confirms how dangerously misplaced her trust has become: Diana orders the call traced and disconnected. The attempt to ask for help allows the assassins to locate and capture her.

And then we lose Coe. He notices Jim, follows the assassin into the restroom and confronts him, but River finds his body after the struggle. Even with the spoilers we had already discussed and the fear that this loss was approaching, watching the moment arrive is heartbreaking. Coe won us over with so few words that his absence feels enormous. There was something about that silence, the way he observed others and acted when necessary, that made him a presence I had already become deeply attached to.

His death also changes the weight of Diana’s negotiation. She hands Sid over on the promise that the others will be spared, but the pursuit is already claiming another life. The agreement fails to provide the safety she is trying to buy, and Coe pays for a conspiracy he had no part in. The escapes, the taunts and the humor remain essential to Slow Horses, but ending with River facing that body turns all the tension into a tangible loss for the group. Knowing it might happen did not make the moment any easier.

Elsewhere in the investigation, Emma Flyte follows Harkness at Lamb’s request and discovers his meeting with Cantor. It is the connection that brings the season’s two major strands closer together: the man pressuring Diana is also working with Frank, who is involved in stealing the Slough House files. This helps make sense of the conspiracy, although it does not entirely dispel the feeling of overlapping operations, with different employers and objectives.

I loved the episode, even as I needed to sort through everything that had happened. The adaptation of Joe Country and Slough House demands attention because the stories still retain their own momentum, but the encounter between Lamb and Harkness shows how much this combination can deliver. The problem is ending with an important discovery for Lamb: Sid captured and Coe dead. The investigation moves forward, but my heart stays behind, in that restroom where River finds another person who did not survive.