Slow Horses Ending Explained: the MI5 Scandal, Donovan, & Did Tierney or Taverner Win?

WARNING: contains MAJOR SPOILERS for series three of Slow Horses

Was series three the best yet for Apple TV’s spy thriller Slow Horses? It was certainly the series with by far the most action, the highest kill count, and the biggest secrets. So… yes, actually.

But while usually Slow Horses is MI5 vs The Bad Guys, this time “the call was coming from inside the house”, and warring factions from within the service were hunting down their own, betraying and back-stabbing all over the shop. 

If you’re looking for a guide to what actually happened to make sure you didn’t miss anything, here’s all your Slow Horses series three questions answered:

What Secret Was Donovan’s Girlfriend Alison Trying to Expose?

While working at the embassy in Istanbul, service agent Alison discovered that MI5 boss Ingrid Tierney (Sophie Okonedo, The Wheel of Time) had signed off on testing a new device that would wirelessly hack computers, with the ability to penetrate closed systems. 

Unfortunately, the device didn’t work, and ended up putting a load of innocent people in hospital, and nearly killed a senior North Korean spy.

The report on this was stored in a file called Footprint. Alison had stolen the file, and was on her way to leak it to the world, but Tierney ordered her to be terminated (i.e thrown off a building in a staged suicide).

What Was Donovan’s Plan for Revenge?

It wasn’t really his plan at all, actually. Alison’s boyfriend Donovan (Sope Dirisu, Gangs of London) had been a grieving mess for a year before he was contacted by an anonymous senior source at MI5 (which we later learn was Diana Taverner – aka Kristin Scott Thomas, Gosford Park) who told him she wanted to help him get justice for Alison.

To do that, he needed to get his hands on the Footprint file, so he could leak it and show the world how ruthless and underhand MI5 really was.

So his anonymous, Taverner-shaped source got him a job for a security company called Chieftain, where she had set up a Tiger Team op: basically, they’d pay Chieftain to infiltrate MI5’s security defences to identify their weaknesses.

Donovan was heading up the op, which involved kidnapping one of the “slow horses”, Standish (Saskia Reeves, Luther), and threaten to kill her if one of the other slow horses, Cartwright (Jack Lowden, Dunkirk) broke into MI5 HQ and retrieved the Prime Minister’s vetting file. Once Cartwright had done this, the official Chieftain plan was to release Standish unharmed and reveal the whole thing had been a security test, but instead Donovan goes rogue. 

At Taverner’s suggestion, he refuses to release Standish, kidnaps another Chieftain worker, and tells MI5 he’ll kill them both unless they let him access the new MI5 archive facility so he can view something called The Grey Books, which are essentially a list of unlikely conspiracy theories that only nutters would want to access.

When he got inside the building, the plan was that he’d be able to find the Footprint file instead, as Taverner had made sure it was there.

What Was Judd’s Involvement in the Tiger Team Op?

To get Donovan inside the archive facility, Taverner needed to convince the powers that be – aka Home Secretary Peter Judd (Samuel West, Van Helsing) – to authorise the Tiger Team op. She convinced Judd for two very good reasons: firstly, she suggests they use his mate’s company Chieftain (in which he had a financial stake) to run the op, and if they’re successful he has a good excuse to pay Chieftain (ahem, and himself) a huge sum of money to act as a security consultancy firm for MI5, and improve their defences.

Secondly, it would embarrass his adversary Tierney, and finally give him the upper hand over her, and MI5 itself.

Unfortunately for Judd, he didn’t know about Taverner and Donovan’s plan, which would ruin Chieftain’s – and his – reputation entirely.

Why Was Taverner Orchestrating Donovan’s Revenge Plan?

We all know Lady Di isn’t exactly the warm and fuzzy type, so she wasn’t helping Donovan out of sympathy for what happened to Alison.

It was simply a power move: Taverner saw a chance to take Tierney down, and she took it.

So she concocted the plan with Judd, knowing he’d be only too pleased to help take Tierney down, and make himself some extra money in the process (see above). 

She placed her ex-employee Spider Webb (Freddie Fox, Year of the Rabbit) as a senior figure at Chieftain by recommending him to the boss there, saying he knew all of MI5’s weak spots. And then she’d put in a good word for Donovan with Webb to convince him to hire him, so he could carry out her Tiger Team plan.

And while Tierney had been quietly moving the Footprint file around so she could eventually just make it quietly disappear as part of their ongoing archiving project, Taverner made sure it was moved to the new archive facility building so Donovan would be able to find it.

The result, if all went to plan, was a double-whammy: while making Judd thinking he was helping get one over on Tierney so he’d help her, Taverner actually used him to help expose Tierney’s scandal in Istanbul, ending her career, and ensuring Judd would have to resign in the fallout appointing Chieftain and letting Donovan go rogue.

Why Were Taverner and Tierney Such Strong Rivals?

There’s been a long-term power struggle between them – Tierney as First Desk stopping Taverner from taking her place, and Taverner as a bitter second-in-command. 

Series three really becomes a war between the two of them: if Tierney stopped the file getting out, she’d stay in command, but if Taverner helped Donovan leak what Tierney had done, she’d be able to take her place as head of MI5.

How Did Tierney Fight Back?

Initially, she thinks Chieftain going rogue is just something she can hold over Judd – it was his idea, and now she can clean it up and regain power over him. So she decides to let Donovan into the archive facility – accompanied by two slow horses, so she could blame them if anything went wrong – and sends MI5 muscle Duffy there to intercept and apprehend them as they leave. 

But when she discovers that Donovan is really after the Footprint file, she tells Duffy that for the sake of the service, there can be no survivors at either the archive facility or the safehouse where Standish is still being held hostage. 

So Duffy and his Chieftain goons set out to kill everyone: Donovan and his accomplices (Alison’s siblings), the slow horses Cartwright and Louisa (Rosalind Eleazar, Harlots), the annoying-but-sweet archive manager Douglas (poor Douglas), and Standish.

How Do the Slow Horses Save the Day?

Jackson Lamb (Gary Oldman, Darkest Hour) finds the safe house and goes to save Standish, so he’s on hand to dispense with the two agents Duffy had sent to kill her (which, rather brilliantly, he does using daft Home Alone-style booby traps). 

Meanwhile slow horses Shirley (Amy-Ffion Edwards, Peaky Blinders) and Longridge (Kadiff Kirwan, This Is Going To Hurt) rush to the archive facility and help take down the Chieftain lot and Duffy, helping Cartwright and Louisa escape. Sadly, it was too late for Donovan and Alison’s brother Ben (Charlie Rowe) who were both killed during the raid.

Has Standish Left?

After Jackson had saved her, he revealed to Standish that her beloved ex-boss (and former head of MI5) Charles Partner had actually been a traitor, selling secrets to the Russians, and was going to set her up for his crimes. She was devastated – and his jibe about how Partner only kept her around because she was too drunk to notice what was going on was admittedly below the belt too. So yes, Standish has officially quit, and the way Catherine walked away did have an air of finality about it, but we imagine Lamb will talk her back around in series four.

What Did River Do With the Footprint File?

He stole them away from the archive facility like the hero he is. Initially he took them to his grandfather (and ex-MI5 agent) David Cartwight (Jonathan Pryce, The Crown) to ask him what to do. To his great disappointment, David sided with the service and said River had to protect MI5’s reputation, throwing the file into the fireplace.

But it’s then revealed that River had given him a fake file, and he leaked the real one, ending Tierney and Judd’s careers for good. 

What Happened to Tierney?

She had to account for both the MI5 killer mission to take out its own agents (which she’d wanted to say “they  tried to steal state secrets and were met with appropriate force”ish ) but also answer to Istanbul. So basically she’s finished.

What Was That Final Conversation Between Jackson and Taverner About?

It’s basically a big dollop of infodumping, but we can forgive them because the rest of the series was so good.

Lamb confirms that Cartwright leaked the file, ending Tierney’s reign, and that she had become First Desk. 

Taverner then casually drops into conversation that she’s “meeting the new Home Secretary” later that day, which confirms Judd is out too.

And Lamb warned her that for someone who had just got everything she wanted, she doesn’t seem very happy… which gives us a good reason to come back and see if she perks up a bit for series four.

Slow Horses series one to three are available to stream on Apple TV+