Slow Horses Season 6 Premiere Finally Reveals Jackson Lamb's Biggest Secret

After a significant tonal shift in Season 5, "Slow Horses" is back to hardcore spy craft and human drama in Season 6. Across five seasons, the show has seen the titular sleuths foil all manner of threat to the British homeland. But throughout it all, one secret has threatened to destroy the longstanding relationship between Gary Oldman's Jackson Lamb and Saskia Reeves' Catherine Standish. Now that secret has come to light in Season 6, Episode 1, and it looks as though things will never be the same.

"Slow Horses" does one thing better than every other spy show: it doesn't dwell on the dearly departed. But one death has loomed over the entire series from the very beginning. Back in Season 1 we learned that former MI5 director general Charles Partner (James Faulkner) was selling secrets to the Russians. Lamb found out and informed his friend, high-ranking MI5 official and future First Desk David Cartwright (Jonathan Pryce). The pair hatched a plan to kill Partner in his flat, making it look like suicide so as to save the Security Service significant embarrassment. In a flashback scene from Season 1, we see that Lamb was the one who broke into Partner's home and pulled the trigger, leaving his former mentor dead in the bathtub. After this, Partner's personal assistant, Catherine Standish, found his body.

Throughout Seasons 1 to 5, though she's learned more details about the killing, Standish has remained unaware that Lamb was responsible for Partner's death. In the Season 6 premiere, however, Diana Taverner (Kristin Scott Thomas) casually lets slip that Lamb pulled the trigger with Standish within earshot. It's a quietly devastating moment for more than one reason.

In "Slow Horses" Season 1, Diana Taverner promises to tell Catherine Standish something she doesn't know about Charles Partner's death, alluding to Jackson Lamb's involvement. She never does reveal that Lamb pulled the trigger, but Lamb himself filled in some gaps without implicating himself, telling Standish he was the one who gave Partner the gun he used to end his life.

Ever since, the truth about Lamb being the killer has remained hidden. During her long stint as MI5's Second Desk, Taverner has used it as a bargaining chip, while Lamb has made it clear he also knows where her bodies are buried. Standish remained none the wiser throughout. In a brutal scene from Season 3, however, Lamb drops a bombshell. "Your hero, Charles Partner, was a f****** traitor," he tells her. "Not even an ideological one. I could almost respect that. But he spent the last ten years of life selling secrets to the f****** Russians." What's more, Lamb reveals that Partner only kept Standish as his assistant because she was a drunk, and wouldn't spot his clandestine activities. On top of that, he was setting her up to take the fall for his treason. 

Lamb stops short of revealing the truth about him being the assassin. In Season 6, Episode 1, a newly-promoted Taverner puts an end to that. During the funeral service for David Cartwright, she tells Lamb, "I'm not Charles Partner. You won't have to shoot me," causing Standish to fall to the ground before fleeing the churchyard. Taverner also says this openly while surrounded by funeral attendees. It's one of the few times Lamb has looked genuinely taken aback, and suggests this new head of MI5 is no longer willing to tolerate his nonsense.

Even when Jackson Lamb told Catherine Standish about Charles Partner being a traitor, he stopped short of revealing that he killed him. In the "Slow Horses" Season 6 premiere, however, Diana Taverner puts it all out in the open. It's a shocking moment, and not only for Standish. Aside from the fact Lamb and Standish's relationship will be irrevocably changed, it signals that in her new role as MI5 First Desk, Taverner is willing to be even more ruthless than before.

Her knowledge of Lamb's involvement in Partner's killing was seemingly one of the main things she could hold over his head. Why would Taverner give up this information in front of Standish and risk her telling others? That would surely undermine much of Taverner's leverage over Lamb. Meanwhile, the entire Slough House staff has been deleted from the MI5 database. Is Taverner finally cleaning house, and revealing the truth about Partner as a devastating parting shot?

Adding an extra layer to this is the possibility that Partner's treachery could have led to a major tragedy in Lamb's past. "Slow Horses" did the unthinkable with Lamb in Season 5, revealing that he was once in love, or as close as a spook can get to it. But he was captured and tortured by the Stasi, during which his pregnant lover was killed in front of him. Was Partner responsible for passing on secrets that led to Lamb's capture and the death of his love? If so, perhaps that might persuade Standish that Lamb was justified in killing Partner. We'll see how things play out, but it looks as though all bets are off this season, and "Slow Horses" remains the best spy show on TV.