Alien invasion movies, a specialised Hollywood domain, works on few fundamental premises -- characterisation of protagonists who the viewers relate to, fierceness of aliens that includes their looks and finally the ability of humans to fight back.
Despite having potential, The Darkest Hour becomes one of the darkest hours of alien invasion movies because it goes wrong on all fundamentals of this sub-genre.
Five young Americans are caught unawares in Moscow after an alien invasion threatens to wipe out every thing that comes in their path. They must work together if they have to survive. But how do you survive an enemy you cannot even see?
One of the greatest reason why alien movies work is the way the writers and the director tease their viewers. This film gets this right. The aliens are not just seemingly invincible, but are also practically invisible.
The other things that work is when, like your protagonists, you are in the dark about what is happening. As the film progresses, you regain your senses to first survive and later to fight back. No such luck in this regard with this film.
Alien invasion films work when the aliens are revealed somewhere after the mid-point of the film and the survivors gather their wits to fight back, even if it is a seemingly losing battle. In this film, the aliens are revealed too late and even then you merely get a scant glimpse of them.
Secondly, there aren`t enough chinks in their armour for humans to have hopes of survival. Hence, the last bit where humanity shows hopes of fighting back, shown here through a radio message, seem concocted and fantastic.
That the writing is so sloppy that not once in the film do you relate to any of the characters or their struggles, only add to the woe of the film. Even a no-brainer action film can work only when the characters and their motivations are believable.
The only good thing about the film is its special effects. The effects are good, but their charm fails because they aren`t backed by a good story to seem believable. Setting the film in Moscow, instead of the usual New York or Los Angeles that have been destroyed countless times by Hollywood, seems to be the only `original` idea in the film.
What you thus have here is a mishmash of different ideas picked up from more films than you care to remember which neither work collectively not on their own.