S1·E2 Episode 2

After the devastating alien attack, disparate pockets of survivors emerge into a shattered world. They quickly realise they must put their differences aside if they are to stand any chance of survival. For they are not alone. Something sinister lurks in the shadows and whatever it is, it won't consider its victory complete until it has killed every last human. But who are these attackers and why are they hell-bent on destruction?