Shock deaths, screw-ups and sleeper cells: here’s everything that’s happened in Slow Horses so far

*Warning: this article contains spoilers for Slow Horses series 1 to 4.

Slow Horses returns for its fifth series on 24 September, and to say we’re excited is an understatement! The BAFTA-winning drama, based on the books by Mick Herron, follows a gang of ‘misfits and losers’ at Slough House, the East London dumping ground for MI5 agents who’ve not exactly made the best impression in the spy service. This gang of agents, however, always find themselves at the heart of the most urgent cases endangering Britain - and, it turns out, they’re not half bad at it.

Each episode has its own meticulously detailed plot, so you’d be forgiven for not recalling exactly what happened in the previous 4 series. And since there are some loose threads that could very well be tied up in the coming instalment, you’ll probably benefit from a little recap!

What happened in Slow Horses series 1?

The first series – based on Mick Herron’s first book in the Slow Horses series, first published in 2010 – kicks off with a training mission at Stansted Airport that ends up being a costly failure thanks to young agent River Cartwright (Jack Lowden). With that, he’s banished to Slough House, the rundown office housing MI5 rejects run by the dishevelled, whiskey-swilling Jackson Lamb (Gary Oldman). Meanwhile, steely senior intelligence officer Diana Taverner (Kristin Scott Thomas) is playing her own game from MI5 HQ, ‘The Park’.

When British-Pakistani student Hassan is kidnapped by a white nationalist group, MI5 swings into action – but the whole thing turns out to be a false flag operation planted by Diana to curry favour with a Pakistani general and make her department look heroic. When it backfires, she scrambles to bury any involvement in the matter. Lamb and River go rogue to locate the captive student and help to save him just in time. Diana manages to keep the incident out of the press by writing off Hassan’s student debt.

Meanwhile, Slough House’s Sid Baker (Olivia Cooke) is shot while surveilling right-wing journalist Robert Hobden. She’s presumed dead, but Roddy Ho (Christopher Chung) can’t locate a record of this at the hospital. We also find out – in a flashback – that Lamb shot Charles Partner, the MI5 officer who Catherine Standish (Saskia Reeves) used to work for. It’s implied that this order came from River’s grandfather, the legendary MI5 officer David Cartwright.

What happened in Slow Horses series 2?

Series 2, which follows Herron’s 2013 book Dead Lions, opens with Dickie Bow, a former MI5 agent, dying on a bus after trailing a man he recognises and leaving the word ‘cicada’ (a term for a Russian sleeper agent) on his phone. River goes uncover as a journalist in the Cotswolds to track down Dickie’s suspected poisoner, infiltrating the flying club of a family – the matriarch of which ends up being, unbeknownst even to her family, a sleeper agent.

Meanwhile, Min Harper (Dustin Demri-Burns) and Louisa Guy (Rosalind Eleazar) are drafted by The Park’s insufferably cocky James ‘Spider’ Webb to sit in on a meeting with an oligarch who could become Russia’s next president. After Min follows the oligarch’s security guards, they kill him, staging his body to look like a cycling accident. The Russians’ plan, it turns out, is ‘Code September’ – a hoax bomb that allows them to quietly loot Russian defector Nevsky’s bank accounts while security is busy elsewhere.

Lamb is at the centre of all this – he’s been targeted by Russian defector Katinsky as revenge for killing an informant years ago. Dickie’s poisoner arrives at the house of David Cartwright – River’s grandfather – to shoot him, but is killed by David. Later, Diana and Home Secretary Peter Judd agree to sweep the whole thing under the carpet.

What happened in Slow Horses series 3?

The third series, based on the 2016 novel Real Tigers, opens in Istanbul, where two MI5 lovers are on opposing ends of Project Footprint, an experimental device aiming to breach air-gapped computers. One plans to blow the whistle by handing over incriminating evidence, but is murdered moments later. A year on, Catherine Standish is kidnapped and River gets a ransom demand that involves him breaking into The Park to retrieve sensitive material. The stunt is soon revealed as a ‘tiger team’ security test outsourced to a private company, commissioned by the Home Secretary.

As punishment for breaking into the archives, River endures a beating from Nick Duffy, head of MI5’s muscle known as the ‘Dogs’. What’s more, River finds out that the tiger team was orchestrated by his arch-nemesis Spider, who now works for private security company Chieftain. But Standish’s kidnappers don’t release her as they’re supposed to, making Chieftain look bad – instead, they want access to MI5’s Grey Books, the documents detailing every conspiracy theory from the past century.

Standish, disillusioned with MI5 herself, helps her kidnappers infiltrate the storage bunker where the Grey Books are kept. River gets a request to help them from First Desk (MI5 director) Ingrid Tearney (Sophie Okonedo) to help them. But she, as it happens, orchestrated the concealment of Project Footprint – and calls on the Dogs to murder everyone in the storage facility. Later, Diana is revealed as the true mastermind behind all this: a plan to topple Ingrid. After a stand-off with Duffy and co, the Slow Horses emerge with the Project Footprint files. Lamb reveals to Standish that her old boss Charles was a traitor who sold secrets to their enemies – and she quits in anger. River leaks the files to hold MI5 accountable, causing Ingrid to resign from First Desk.

What happened in Slow Horses series 4?

This series, adapted from 2017’s Spook Street, sees a car bomb going off at Westacres shopping centre in London. David Cartwright, now battling dementia, is startled when an impersonator pretending to be River tries to kill him in his house. Instead, David kills the assassin, and the real River heads to France to track down the assassin using his passport, hiding David at Standish’s house. This hunt leads him to Frank Harkness, an ex-CIA mercenary who grooms child soldiers.

After piecing together some evidence based on old birthday cards, River learns a horrible truth: that Frank is his biological father and he’s linked to the Westacres bombing. Frank’s gunman Patrice hijacks a car with River in it and kidnaps him, bringing him to meet Frank in a restaurant – who then tries to recruit him as a mercenary. River, naturally, rebuffs the offer. Meanwhile, Patrice kills Sam Chapman – an old associate of Lamb’s – as he could connect Frank to the bombings.

The Dogs, now headed up by Emma Flyte (Ruth Bradley), rush to the restaurant to find Frank, but he makes a cunning escape by using a grenade as a bluff. River finds him at a train station and he surrenders to arrest. Patrice enters Slough House and Marcus Longridge (Kadiff Kirwan) is collateral damage, much to the horror of his partner Shirley Dander (Aimee-Ffion Edwards). River moves David into a nursing home, and Frank threatens to release some documents proving he’d been hired by MI5 over the years to carry out their dirty work. We don’t see it, but we presume he’s been set free.

How can I watch Slow Horses series 5?

There’s not long to go! As mentioned, the series will premiere globally on 24 September 24 with the first two episodes, followed by one episode weekly until 22 October. While you wait, you can stream series 1-4 now on Apple TV+.