There's nothing quite like a movie that turns out to be great in all the respects you thought it'd be rubbish in. An end of the world disaster flick starring Gerard Butler? I expected Gods of Egypt-level dreck in the performances, script, effects and everything else, and if the delectable Monica Baccarin hadn't been in it I might not have bothered.

But her and the promise of some big screen destruction (another one that ironically went straight to the small screens of streaming services because of the COVID19 pandemic) made me cue it up, and I haven't been as glad I did for almost any other movie this year.

As the trailer promises, it's the story of a family fleeing the effects of a huge comet heading towards Earth as meteoroids rain down from the sky in various set pieces, and you expect nothing more than hokey family values and Roland Emmerich-style skin-of-teeth escapes (I even expected a dog).

There's none of that. It's even - dare I say - as realistic as you've ever seen a B grade end of the world movie. Every beat, emotion and reaction is relatable and feels the way average people would really act, even one man gritted-teeth-clenched-scream Butler. The least plausible thing in the whole movie is that any man married to a specimen as delicious as Baccarin would cheat on her.

Shut Up Buttwad is everyman engineer John, estranged from his wife Allison (Baccarin) but who still desperately loves their son Nathan (Roger Dale Floyd). As he arrives at their house and you see the strained relations between he and Allison, everyone is talking about Clarke, a comet that's due to pass close by to Earth and provide quite a show for humanity.

But while he's at the grocery store collecting provisions for a birthday party, John gets a text from the emergency services informing him that he, Allison and Nathan have been selected to be taken to an emergency shelter, Clarke apparently more of a threat than the government has let on.

They all watch the news in terror as a fragment hits and annihilates a city in Florida, the horrific shock wave hitting their suburb moments later and a red mushroom cloud covering half the sky, revealing the severity of the situation to everyone. Apologising to their panicked neighbours (by this time another alert has appeared on their TV, instructing them to make their way to a nearby air force base, so the cat's out of the bag), they pack their bags as instructed, jump in their car and hightail it.

But they have to leave the car on a jammed freeway and run for the base where they find chaos as terrified crowds have descended, harried soldiers at the gates trying to only let those who've been cleared for travel and have the special wrist bracelets inside.

But once John, Allison and Nathan are in the processing area, another disaster strikes when they realise Nathan's insulin has dropped out of his bag in the car. As John tries to fight his way through the crowds and bureaucratic melee to get it and return to the base, things get worse for Allison and Nathan as the army apparently had no record of their son's illness, which invalidates them all from going.

It starts a chain of cause and effect that splits the family up and blows them on the winds of fate across several states, all of them in terror at not being able to find each other, and it's the first surprising pleasure of the film that the plot mechanics by screenwriter Chris Sparling (The Atticus Institute, which I also really liked) of them getting separated and finding each other again are entirely believable. There's no mum and dad just happening to drive past the waterlogged ruins of San Francisco in a boat and the daughter they're separated from getting their attention from a nearby building with a laser pointer in the middle of the day here (yes, that was the ridiculous San Andreas).

Better still, you feel every knot of frustration, clutch of terror and swell of elation they do as they're split up and reunited because of the honesty of the performances – even in Butler. Even while he kills a guy defending himself there are no rippling muscles or snappy one liners. Everyone (even the archetypes) are fully realised, very real people.

And in amongst all that is the shadow of the huge interstellar object coming to kill us all. The fact that it's heading for Earth and has been designated extinction-level can't be kept secret anymore so as Allison, John and Nathan try to find their way back to each other and their ultimate destination (which finally reveals the truth behind the cryptic title of the movie) society crumbles around them.

Even when it enters real sci-fi territory and we learn a piece of the comet big enough to wipe out human civilisation is coming and it becomes a race against time for the family to reach the military bunker representing the last chance for a few survivors, the swelling soundtrack and palpable fear on everyone's face makes it as much an effective drama as an adventure thriller romp.

Greenland is proof again that you should never judge a book by its cover.