The five F's in Slow Horses, explained: Lamb's list only has four — and he's still right

The sixth season of the Apple TV spy series ran from September 16 to October 21, with Gary Oldman as Lamb, the head of Slough House, MI5's dumping ground for failed agents. The F's arrive in episode 2, "Daddy Issues." Two former slow horses, Struan Loy and Kay White, are dead in what were made to look like accidents, Slough House's records have been wiped, and Lamb has to work out who's next.

The four on the list

Lamb gathers his team and announces: "We need to address the five Fs." Then he names them:

• Who the f***?
• What the f***?
• Where the f***?
• And why the f***?

They're the four questions of any investigation, in Lamb's vocabulary.

The fifth

J.K. Coe, played by Tom Brooke, the slow horse who rarely says anything, points out the obvious: "That's four." Lamb doesn't miss a beat: "F*** off. That's five."

The fifth F isn't a question. It's what Lamb says to Coe, and it completes the list in the act of saying it. Lamb isn't correcting himself; he's demonstrating the fifth item.

The first four F's are how you investigate. The fifth is how Lamb ends things.

In Mick Herron's novel Joe Country, which this season adapts along with Slough House, Lamb sticks to the honest count: "As with any mystery, you have to address the four Fs." The show added the fifth.

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How the season proves him right

The four questions do get answers. The killers are Russian assassins sent after anyone connected to Slough House. Their target was Sid Baker, whose identity was used for an unofficial MI5 assassination in Kazan that Diana Taverner ordered and paid for with a media tycoon's money. When Diana erased Sid's records, the Russians followed the trail to Slough House.

But answering the questions doesn't stop anyone. What ends the threat is the fifth F, aimed at everyone at once:

• The Russians — Lamb sells the official running their clean-up, Natasha Carrington, a story in which the tycoon, Damien Cantor, was a double agent, and hands Cantor over as the man behind Kazan.
• Whitehall — Diana tells the Home Secretary that Peter Judd was behind it, and keeps her job as First Desk.
• Frank Harkness — Lamb leans on Cantor to call off his hired gun, who has been cleaning up loose ends after the theft of Slough House's files.

Lamb gives the Russians someone to blame and Whitehall a story, and both go away.

Coe, the only one in the room who counted, doesn't make it to the end of the season. He's killed by Jim, one of the two Russian assassins.

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