The opposite of an escapist blockbuster, the eighth and apparently final outing for Tom Cruise's Ethan Hunt is the doomiest and gloomiest yet in the action-adventure franchise.

With so much tension and conflict around the world, it can be a relief when a Hollywood blockbuster distracts audiences with some escapism, some optimism, and some rollicking, good-natured fun. Mission: Impossible – The Final Reckoning is not that kind of blockbuster. The eighth instalment in Tom Cruise's globe-trotting action-adventure franchise, The Final Reckoning is a miserable, apocalyptic tract which is fixated on the subjects of how close we are to nuclear armageddon, and how quickly civilisation can collapse. Yes, you get to see Cruise having a fight in his underpants, and doing another of his hanging-off-a-plane routines, but even so, it could be the feel-bad film of the summer.

"Truth is vanishing, war is coming," someone intones at the beginning of the film, and then we're subjected to shots of missiles launching and cities being obliterated. In place of snappy banter, there is cod philosophy about destiny and choice, and in place of Lalo Schifrin's adrenaline-pumping classic theme, there are orchestral minor chords on the soundtrack. What's disappointing about all this doom and gloom is that the franchise has made the kind of whiplashing U-turn you might see in its car-chase sequences. The last Mission: Impossible film, Dead Reckoning, was a funny, frothy Euro-caper sprinkled with mischief, glamour and romance – or as close to romance as you're ever going to get in a Cruise production – and the follow-up has the same writer-director, Christopher McQuarrie. Yet The Final Reckoning, set almost entirely in tunnels and caverns, and in the depths of the ocean, is the dullest and darkest film in the series, both literally and figuratively.

It devotes an inordinate amount of its almost-three-hour running time to scenes of people sitting in shadowy rooms, explaining the story to each other in gravelly whispers. Again and again, we have to sit through these ponderous, portentous mutterings: the title might as well have been Exposition: Interminable. Usually, these scenes are punctuated with flashbacks to what's happened before, flash-forwards to what might happen in the future, and flash-sideways (if that's a term) to different people, in different shadowy rooms, explaining the same story in the same gravelly whispers. But instead of livening up the exposition, this........

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