A survivalist horror movie starring a suitably gaunt and ravaged-looking Halle Berry as a mother intent on protecting her twin sons (Anthony B Jenkins and Percy Daggs IV) from the evil that stalks the world outside, Never Let Go is a slippery film seeded with doubts and uncertainties. The one thing we never question, however, is that there is a very real threat to the children’s safety. Whether that threat comes from something malevolent lurking in the woods, or whether it’s located closer to home is key to the film’s sinuous sense of menace. The picture, directed by French genre whiz Alexandre Aja (Switchblade Romance, The Hills Have Eyes), is reasonably effective up to a point – it taps into the hostility of the natural world and the rituals that we adopt to keep ourselves safe. But this kind of horror storytelling is only as successful as its final act. And, unfortunately, Never Let Go drops the ball, along with the bloodstained machete, just when it should be ramping up the tension.

• In UK and Irish cinemas