Set on one fateful night in South London, the story begins when a gang of teenagers, led by the stoic Moses, mugs a young nurse named Sam. Their crime is interrupted by a meteorite crashing into a nearby car, releasing a small, aggressive alien that Moses kills. However, much larger, "gorilla-wolf" creatures soon begin falling from the sky in its wake. The teens must then retreat to their high-rise apartment block—the "Wyndham Tower"—transforming the low-income complex into a fortress to defend their community.

Director Joe Cornish was inspired to write the film after being mugged himself; he noticed his young assailants seemed as scared as he was and wanted to explore their perspective through a sci-fi lens.

To keep the creatures terrifying on a modest budget, the "aliens" were largely portrayed by performers in suits, which were later enhanced with CGI to make them "blacker than their own shadows".