Look at it this way: at 10.15pm tonight, we will all be able to breathe again. By then, the final episode of BBC1’s Bodyguard – the propulsive political nailbiter from Jed Mercurio that has blown up in the public imagination over the past month – will have galloped towards its conclusion, even if no-one can quite agree exactly what that might be.

Partly, that’s because Mercurio, the veteran TV writer behind Line of Duty, Cardiac Arrest and The Grimleys, has so confidently front-loaded his obsessional six-parter with the sort of character crises, large-scale action scenes and audacious plot twists that lesser dramas might save for their finale.

When you unexpectedly kill off one of your two leads halfway through the series, where else is there to go? (A polite warning: there will be some spoilers for the first five episodes ahead.)

Bodyguard – starring Richard Madden as traumatised squaddie-turned-protection officer David Budd and Keeley Hawes as Julia Montague, the hawkish home secretary under his wing – certainly hit the ground running. Its first two instalments screened back-to-back on a Sunday and rest-of-UK Bank Holiday Monday, with the first episode hitting an impressive high of 6.9 million viewers. Since then, it has only gained momentum, adding further millions via iPlayer and, crucially, growing the number of people who tune in live on the night of broadcast. At its peak, last Sunday’s fifth episode had 8.3 million on the edge of their couches trying to unravel the tangle of conspiracies. Consolidated catch-up figures will push it past 12 million viewers, an astonishing figure for a drama (for context, that’s far better than recent X Factor finals and only a few steps behind Strictly).

For the BBC, currently fighting a domestic battle against moneybags global streaming services like Netflix and Amazon, the wildfire success of Bodyguard has been positioned as a triumph of homegrown pluck, an unexpected victory for appointment TV in an era of listless but moreish bingeing.

The corporation issued a press statement confirming that Bodyguard was the biggest debut drama launch “across all UK channels” since 2006 (the title had previously been held by ITV’s Morse spin-off Lewis). Even in a particularly strong autumn

BBC drama – including the powerful IRA bombing standalone Mother’s Day, the racy but emotionally weighty Wanderlust and the enjoyably oddball Killing Eve – Bodyguard has towered over everything else. It has even served an unexpected protective function, inspiring so many admiring column inches in the media that it has undoubtedly saved Press, BBC1’s torrid Thursday night tale of feuding newspapers, from even more of a mauling from cynical hacks.

Trying to reverse-engineer the secret of Bodyguard’s success – as, presumably, many other writers and production companies are already trying to do – is trickier than it seems. While Scottish star Madden previously played Prince Charming in Disney’s Cinderella, it is not a fairytale story about a surprise hit that came out of nowhere.

Mercurio’s reputation as a creator of supremely engrossing thrillers has grown in tandem with the success of Line of Duty, his addictive procedural about the (fictional) AC-12 police squad tasked with collaring bent coppers.

MERCURIO’S enviable talent for trans-forming long and potentially rather dull scenes in plausibly dingy interrogation rooms into masterclasses in tension – as well as Line of Duty’s migration from BBC2 to BBC1 for its fourth season – presumably all served as proof-of-concept for Bodyguard.

If it had been pitched by someone other than Mercurio, there would likely have been some executive pushback at the creative decision to open the series with an extended 20-minute sequence centred around a suicide-bomb threat on a packed passenger train trundling towards Euston. In cop lingo, Mercurio had previous, so was sanctioned to run with it.

That nerve-shredding opening sequence – where an off-duty David patiently talks down a female suicide bomber while his two kids peacefully doze in an adjacent carriage – upended most of the hard-and-fast rules of thriller storytelling (which basically boil down to: hit ‘em hard, and hit ‘em fast).

Mercurio further messed with established rhythms and expectations by staging a sniper assassination attempt midway through episode two and essentially killing off Julia with a bomb blast in episode three.

With the abrupt removal of its co-lead, Bodyguard suddenly morphed from a tense two-hander into a one-man show centred around David and his quest to uncover the truth.

If audiences suspected the traumatised protection officer might actually be a Homeland-style “baddie” – a PTSD-suffering soldier so aggrieved by the political readiness to send men to war that he might bump off the person he was duty-bound to protect – he is now fairly obviously the “hero”, if only by default.

The show seems to value realism, in that there is no shortage of official-sounding acronyms being bandied about, like PPO (Personal Protection Officer) and PNC (Police National Computer). But Bodyguard also seems to exist, almost reassuringly, in a pre-Brexit political world, one where most of the politicians actually seem strong-willed and marginally competent, even if what they are doing is pretty horrible.

While it may feel dense with detail, Bodyguard is also a sleek show that seems most alive moment-to-moment. Mystery box dramas such as HBO’s Westworld force fans to pore over paused screenshots or associated websites to try to disinter clues to its mysteries. Bodyguard simply tosses out lots of shady characters and murky motivations but focuses on sustaining momentum.

It is something to experience rather than something to compile a dossier on, which might explain why live viewers have cleared their Sunday night schedules to join the ride.

Recent episodes have, inevitably, steered hard into conspiracy theories. If Julia was colluding with MI5 to overthrow the PM and push through a new snooper’s charter, why would the secret service take her out? Could the Met – represented by the brilliantly icy Gina McKee as Commander Anne Sampson – really be so paranoid about their jurisdiction over terror threats that they would plot to kill a home secretary?

What are the explosive secrets contained in the digital tablet loaded with kompromat – that poetic but suitably Russian term for compromising personal material primed for blackmail – that David now has in his possession?

What risks being lost in the shuffle, I think, is the other fuse that ran through those first episodes. It was not one attached to a suicide-bomb vest or a truck crammed with explosives veering towards a primary school. It was the intense chemistry between Madden and Hawes.

Their romantic relationship was unlikely, and not just because it broke David’s Royalty and Specialist Protection Branch protocol.

EVEN while anticipating the next terror threat, audiences might have felt another tingle as the stoic, furrow-browed Scot and the posh, poised politico finally addressed the haywire chemistry between them and embarked on their forbidden affair (accelerated, of course, by the fact they had both just survived a life-or-death sniper ambush).

That sexual tension really revved up Bodyguard, a vital charge that has kept it barrelling forward.

For Hawes, the whole experience may end up feeling like a sneaky half-day. She has enjoyed all the benefits of being promoted as the lead character in a BBC1 primetime drama while getting to bunk off after three episodes.

How many other actors have merited a Radio Times cover after they have left a show? Hawes previously starred on two seasons of Line of Duty for an arc where her character was also unexpectedly killed off, suggesting she trusted Mercurio and knew what she had signed up for as Julia.

While some hardcore Bodyguard fans maintain that her character will return as a jaw-dropping twist, Hawes has insisted her haughty home secretary has shuffled off this mortal coil, run down the curtain and joined the choir invisible. (Although when Newsnight recently sent out a Twitter call for questions to be put to Mercurio ahead of the finale, Hawes gamely jumped in: “Is Julia Montague alive?”)

For Madden, it has been even more of a career rocket-boost. The Renfrewshire-born actor was a key part of Game of Thrones as Robb Stark in the early seasons of the downbeat fantasy saga, a proud King in the North who turned out to be too much of a straight arrow to survive in George RR Martin’s down-and-dirty realm of sharp-edged realpolitik.

After a cut-glass turn as the Prince in Cinderella and showcasing an American accent in the Paris-set thriller Bastille Day (aka The Take) Bodyguard has allowed him to lean heavily into his Scottish brogue.

David’s PTSD and detail-obsessed approach to his job also means any stiffness in Madden’s performance simply reinforces the sense of a character attempting to control his trauma by throwing himself into work.

Even if David heroically sacrifices himself in the finale that might not necessarily be a bad thing. Madden’s stock has never been higher – he has jumped from 20/1 to 14/1 to be the next Bond – and if Budd buys it, he will be free to move on to other projects.

So: a triumph for the BBC and one in the eye for streaming services? Except, like many a Mercurio production, there is a twist in the tail.

The most popular BBC drama in a generation is actually made by ITV, who acquired a majority stake in World Productions, the company behind Bodyguard and Line of Duty, in 2017. Netflix were also tangentially involved at the development stage and have since secured the international rights to show Bodyguard in the 189 countries that it streams to other than the UK.

Even the fact that tonight’s finale will be an extended 75 minutes feels like a concession to the ascent of streaming, those flexible platforms where creators are not required to fit into the predetermined, essentially archaic timeslots of “normal” telly.

So what initially looks like the BBC thumping ITV on weekend primetime might eventually look more like Netflix getting one over on Amazon, who co-produce Vanity Fair, ITV’s wobbly Sunday night competition for Bodyguard.

Such is the labyrinthine reality of TV production in 2018, as tangled and yet intertwined as the various antagonistic factions in Bodyguard.

Graeme Virtue is a freelance writer based in Glasgow. You can follow him on Twitter @GraemeVirtue