He is shipped to "The Gorge" a giant mist-filled crevasse. He must crew the western watchtower and maintain the significant automated defensive perimeter, alone for a whole year. He's not there to keep people out but to keep what's inside from getting out.
His opposite number in the east watchtower, Drasa (Anya Taylor-Joy), is there to do the same on the behalf of Russia: this is some post-WWII Great Powers joint project that has survived the Cold War and the fall of the Soviet Union.
Communication across The Gorge is forbidden but as the months roll by boredom takes its toll and the pair start a weird long distance, drawn out flirtation.
No good will come of this.
CW: some body horror imagery
Scott Derrickson does a bit of a bait and switch for anybody expecting pure action; a big chunk of this is the extended meet-cute between the pair of badasses.
I'm there for that though, it works really well.
Inevitably they end up in The Gorge and here it loses its way a bit.
The action is OK and the creature design and scenography are nicely grungey and reminiscent of the mutated horrors in The Upside-Down/Annihilation. The conspiracy revealed as they explore is decent in principle but they fill in the detail with some clunky staged exposition that grates. Then after briefly depowering the previously awesome Drasa so she can be rescued we're into very generic 'flee the hordes' action.
Overall it wasn't what I expected until suddenly it was and makes for a reasonable bit of lightweight entertainment 7/10.
I wish they never actually went into The Gorge. The initial setup is that both are weary of what they do and having them work together to unpick the same story from outside looking into the mists would have been far far more interesting but obviously hard to sell. In my head I can make this work but well I'm not a filmmaker. Disjointed information on either side of The Gorge that when combined has the solution. It would be love and co-operation conquers all nonsense but more unusual nonsense than 'shoot the monsters'.