Every great thriller franchise eventually has to answer a question it would rather avoid: what was the show actually about? For Vigil, the honest answer was never Suranne Jones, or the murder board, or even the Royal Navy. It was the box. The first run of the BBC drama sealed two detectives and a dwindling supply of oxygen inside a nuclear submarine and let the walls do the work, and it belonged to a lineage as old as the form — Hitchcock’s Lifeboat, 12 Angry Men, Das Boot, every story that discovers character by refusing anyone the exit. Vigil was a bottle thriller wearing a police-procedural coat.

That is worth remembering now, because season three is about to leave the bottle behind for the widest space it has ever tried to fill. Jones’s DCI Amy Silva and Rose Leslie’s DI Kirsten Longacre are being sent to a remote research station in the Arctic, where a member of a covert British special-forces mission has been shot dead and a small murder inquiry keeps threatening to detonate into something between nations. The publicity calls it the pair’s most high-stakes mission yet. The more interesting stakes are the show’s own.

Because Vigil has run this experiment before, and the results are on the record. When the second series climbed out of the submarine and relocated to an airbase in the Middle East, something quietly leaked out with the cabin pressure. The audience nearly halved; the reviews cooled from admiring to merely polite. One verdict has clung to that season like frost: taking Vigil out of its submarine relieved the pressure, and most of the suspense along with it. The desert was vast and sunlit and full of exits, and a thriller built on having nowhere to run suddenly had somewhere to run.

So the Arctic is not a neutral change of scenery. It is a wager on the very thing the franchise mislaid — and here the comparatist in me perks up, because a frozen research station is not an open setting at all. It is one of the screen’s most reliable pressure cookers. This is the terrain of The Thing, of Fortitude, of a hundred stories in which the cold outside does the sealing that steel bulkheads used to do. Svalbard, in the right hands, is a submarine you can see the sky from: no help coming, no road out, everyone trapped with a killer and their own paranoia while the dark closes in for months. Get that right and Vigil doesn’t merely recover the claustrophobia of series one — it upgrades it.

The worry lives in the pitch. Writer and creator Tom Edge has described the season as a story about melting ice caps, resource wars and lives spent in the pursuit of peace and profit — a geopolitics of the pole rather than a chamber piece. That is a serious, of-the-moment subject, and it is also precisely the register in which the second series went slack: the bigger the map, the thinner the tension, because a conspiracy that spans a region cannot press on your chest the way a sealed corridor can. The great isolation thrillers stay claustrophobic even when their theme is enormous; the weak ones mistake scale for stakes.

None of which is reason to write the season off — rather the opposite. Jones and Leslie remain one of the most watchable double acts on British television, the supporting bench is deep, and Edge has shown before that he can turn a technical briefing into a nightmare. The single-location instinct that made the show is even printed into its title: a vigil is a watch kept in the dark, and there is no darker watch than an Arctic winter. The raw material for the best season yet is all there.

The particulars sit where this show’s logistics always have, in the background until they matter: six one-hour episodes on BBC One and iPlayer, with Gary Lewis, Dominic Mafham and Orla Russell among the returning faces and a large new ensemble filling out the station, written by Tom Edge for World Productions and shot between Scotland and the real Svalbard rather than a soundstage doing an impression of it. It arrives at the end of August.

All of it hinges on a decision the first-look images cannot reveal: whether Edge treats that research station as a bottle or a backdrop. Get greedy with the landscape and you have prestige tourism with a corpse in it. Keep the doors shut and let the ice do what the ocean once did, and Vigil comes home — to the one place it was ever truly itself, which was nowhere to go.