Slow Horses Hinted at Season 4 Twist in Hidden Easter Eggs

Warning: contains Slow Horses season 4 plot spoilers.

As The Usborne Official Spy’s Handbook teaches, powers of observation are crucial to spycraft. The makers of British spy thriller Slow Horses have been testing those powers in its audience by including background details teasing major plot twists.

In Slow Horses season 4 episode 4 “Returns”, Jack Lowden’s character River Cartwright is chased through St Pancras train station by Park Head of Internal Security Emma Flyte and two of her “dogs”. River evades capture by escaping onto a tube train, the doors of which close before Flyte and co. can board. He gets away, and the others gnash their teeth in frustration on the platform.

Behind them as they do so are a trio of tube station posters: one advertising a festive King’s Cross art fair, one advertising an online genealogy service, and one advertising a new production of a Shakespeare play. So far, so typical. Except that those posters appear to be the work of Slow Horses’ art department, and feed specifically into season four’s plot.

The fictional genealogy service poster is illustrated with the famous “March of Progress” image of humans evolving from great apes, and the tagline: “What’s Your Story?”, and hangs next to a poster for Hamlet decorated with a skull and described as “A Ben Taylor production”. (No such theatre producer or director appears to exist by that name, but a poster-designing artist does – perhaps a coincidence, perhaps not.) They both appear at a point in the season when the audience has enough info to piece together season four’s big plot twist: the fact that Frank Harkness, the villain River Cartwright encountered in France, is River’s biological father.

30 years earlier, River’s mother had been used by the rogue CIA agent, seduced and impregnated, so that Frank could trade her back to her father – MI5 bigwig David Cartwright – in exchange for valuable assets.

In that light, the posters’ thematic relevance becomes clear: ancestral origins and Shakespeare’s most famous play about father-son power dynamics? It’s River’s season four story.

That scene isn’t the first time we’ve seen an ad for the (fictional, as far as we can work out) King’s Cross Winter Art Fair either. In episode three, Jackson Lamb’s stolen Hackney cab drove by a bus advertising the same event. As well as setting the early-January scene, the poster teases the site of River and Frank’s dramatic finale meet-up at Granary Square, King’s Cross, and the chase through the train station.  

It’s not the first time that Slow Horses’ art team have used a London bus ad to inform their story. In episode two of season four, Kristin Scott Thomas’ character Taverner takes her new boss Claude Whelan onto the top deck of a bus to have an unobserved conversation. That bus sports an ad for a fictional exhibition on “Great Leaders” with the tagline “500 Years of Defining Authority”. The satirical dig at Whelan is clear – the character is anything but a great leader, and the latest in a long line of real-world underperforming and self-serving politicians promoted far above their abilities. Illustrating that exhibition is what looks like a Holbein portrait of King Henry VIII – a man whose personal weakness and vicissitudes threw England into divided chaos -, and perhaps intended as another swipe at some of Henry’s more recent political counterparts in the real world.

While we’re spotting season four background details, it’s worth pointing out a difficult-to-see cameo but one officially confirmed here from Mick Herron, author of the Slow Horses books on which the series is based. Joining Herron’s season three cameo in a Chinese restaurant scene is this one in which Herron and his partner flag down a taxi outside the fictional Orianne Hotel – the swish new palace that now occupies the site of old HQ from David Cartwright’s days.

Any more eagle-eyed spots that we’ve missed, all welcome!

Slow Horses season four is available to stream now on Apple TV+