In 1977, the Alpine Heights tower block where Kyle Dempsey and his sister Lauren live with their mum and stepdad becomes the site of a horrific infestation. Believing their stepdad, Connor, to have been killed in action in Northern Ireland, Kyle and Lauren are shocked to find him at home in their flat. But Connor is changed. Following a secret military experiment, he has been infected with some kind of alien life form, which is using him as a host to infect the other residents of the tower block with gruesome results.