Apple TV's 'Slow Horses' closed its sixth season with 'Judgement Day,' and the final image is the one viewers are talking about: River Cartwright, back at his grandfather's house with a bottle of red in hand, ready to finally pour his heart out to Sid Baker — whose note led him there — collapses with an apparent heart attack before he can say a word.

The last thing shown is his foot twitching through a doorway, then nothing.

Vulture, which rated the episode five stars, calls it a cliffhanger that throws Slough House's future into question, and Esquire's writer admits he is holding off on the black tie, remembering Season 4 when River planted his wallet on a corpse and played dead.

The apparent death lands as a genuine shock because it interrupts the season's rare quiet moment. Vulture notes Sid kept her promise from the previous episode, leaving a light on for River — a reference to the story River told her about how his grandfather always signaled he was home.

IGN's Leanne Butkovic, scoring the episode a 7, says the show raises a ton of questions: did Sid lure him there, or did Jim and Jane plant poison on the crowbar River handled in Episode 3, getting their job done from beyond the grave?

IGN admits that theory seems too obvious, but says it is still prepared to be made a fool of by the show again when Season 7 arrives next year.

The cliffhanger is set up by a brutal Suffolk siege. Esquire reports that River's father, Frank Harkness, arrives with two new henchmen to storm Taverner's holiday home, turning it into what the column calls the Alamo in cable-knit jumpers.

Harkness shoots ex-Dog Emma Flyte dead after she heroically takes one henchman with her, while Louisa survives a gut wound.

Back at the house, Jane ambushes River, but Harkness kills her with a single shot rather than kill his own son, then quotes Larkin in a father-son reunion before slipping out the back as rescue choppers arrive.

Vulture highlights his line, 'Everything changes shape,' as key to understanding him — and a warning to River.

IGN reports that Reece Nesmith gets his hero moment, pulling off a Chekhov's harpoon — firing a salvaged harpoon into a henchman's back just before Roddy is shot — a beat Esquire describes as ending with the man crumpling to the sand.

Meanwhile, Gary Oldman's Jackson Lamb enacts his scheme at a Russian child's birthday party, where Vulture notes Natasha's daughter mistakes the disheveled spy for a clown.

Lamb pitches Natasha on selling Moscow a fictional double-agent mastermind behind the Kazan hit, framing Cantor's own dirty money as the paper trail that damns him.

Vulture calls the humbling of Diana Taverner the season's major takeaway.

In what it deems the most gripping scene, Kristin Scott Thomas's First Desk — who spent the season quadrupling down on denials — confesses to Lamb, 'I am the greatest threat to national security we have seen in years, and I'm taking myself off the board,' before Lamb throws her a lifeline after demanding she get off her cross.

She scapegoats both Cantor and Peter Judd, and Harry Lloyd's media baron is flown to Russia to face what Esquire calls a grim fate. IGN, however, finds Taverner's turnaround a stretch and Cantor a one-note villain whose media influence was barely shown.

IGN reports Season 7 is due next year, and urges fans to start cranking theories about what exactly took River down, noting the teaser appears to be Shirley- and Ho-centric and that Taverner may still face consequences.

Vulture sees unresolved threads everywhere: Taverner's issues with her Russian counterpart Vassily are not over, Sid's ambiguous motives remain unexplained, and the poisonous residue on the crowbar — which River made a show of in Episode 3 — is the likely culprit, if an obvious one.

With Emma Flyte dead, Coe gone, and Olivia Cooke's Sid given only three seconds of distant screentime per IGN's complaint, Slough House has a reshaped roster to rebuild around.