The film was produced by Nira Park and James Wilson, with Edgar Wright serving as an executive producer.
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.
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.