How very unexpected. The Christmas movie of 2017 has turned out to be a cheerfully unpleasant and bracingly nasty horror film: an ingenious and efficient black-comic shocker unfolding in real time over one seasonal evening in a prosperous American home. The twists and lurches are interspersed with bouts of boisterous carol-singing. It looks in many ways like a garden-variety home invasion drama, with hints of Home Alone and Funny Games. But there are suggestions of something older, like a well-made stage play. Dial S for Slasher?

Levi Miller plays Luke, a sensitive, somewhat pampered 12-year-old with a major crush on his babysitter Ashley (Olivia DeJonge), who has come round to look after him while his parents – amusing cameos for Virginia Madsen and Patrick Warburton – are out at some yuletide bash. Naughty Luke has been swigging booze, and he has put a horror film on the home entertainment system, apparently hoping Ashley will cuddle up to him on the couch in fear. Then a dark shadow appears at the window, the phone starts ringing and no one’s there.

It’s enjoyably brash, an exhilarating mix of elegance and yuckiness.