Slow Horses Season 5 Is Based On The Twist-Filled London Rules

If you’re already swept up in the Emmy-winning phenomenon that is Slow Horses, you know that each season of the Apple TV+ hit follows a different book in Mick Herron’s series about a group of misfit M15 agents. Slow Horses Season 5 (which premiered on Sept. 24) is based on London Rules, which covers the suspenseful, twist-filled investigation into a string of terrorist attacks that have a surprising connection to the underdog unit.

Want to take a peek ahead at this season’s mystery? Here’s the London Rules book ending and plot summary, explained.

A Violent Pattern

London Rules begins with a mass shooting in a small English village. Soon after, more violence occurs, including the bombing of a zoo penguin enclosure and an attempted explosion on a train.

At the same time, it seems that someone is trying to kill Slough House member Roddy Ho. J.K. Coe works out that the seemingly random violence is connected to Ho’s relationship with Kim. You’ll remember that Kim was introduced as Ho’s “girlfriend,” really a bot, in Season 4 of the show. But in the book, she’s a real woman who uses Ho to access MI5 secrets. Specifically, she secures a document outlining the agency’s blueprint for destabilizing countries. As Coe explains, Kim’s group is using the system against the very country that created it.

Realizing that the next step in the document is the assassination of a populist leader, the team sets out to stop it from happening. However, in trying to help River Cartwright in a skirmish with a suspected would-be assailant of Dennis Gimball, a member of parliament (MP), Coe knocks over a paint can that ends up killing Gimball.

The Slough House Connection

While this wasn’t the terrorists’ doing, they move onto the next stage in the blueprint: seizing control of the media. They do this by returning to the site of the first attack, where a memorial service is being held. Coe correctly predicts this and works with Shirley Dander to help fight the attackers.

As Jackson Lamb explains, it was Claude Whelan’s Operation Shopping List — a scheme to disseminate misinformation to enemies — that led to the declassification of the blueprint nearly 20 years ago. Because of the way it backfired, Lamb says, he will likely be ousted, making way for Diana Taverner to replace him as First Desk.