Three years ago, director David F. Sandberg made a great short about a woman home alone at night, noticing a dark figure when she switched the light off. The payoff was both hilarious and scary as shit. So, of course, producer James Wan got a hold of Sandberg and now there’s a full length feature film based on that light-switch premise.

Writer Eric Heisserer takes the idea, fleshes it out, and comes up with a pretty good story to go with Sandberg’s strong horror directing abilities. Rebecca (Teresa Palmer) is an angry woman with mommy and commitment issues. Her mom, Sophie (Maria Bello), recently lost her husband and has fallen into a depression where she is talking to herself. Her son, and Sophie’s brother, Martin (Gabriel Bateman) is seeing a strange dark figure when the lights go out. It all leads up to a finale where flashlights are very valuable and potential victims behave like idiots.

Sandberg repeats the same jolt scare over and over again, and makes it all work nicely. The film is genuinely scary in the moments it’s trying to be. The background story is a little on the flaccid side, but Palmer and Bello are good in their roles, and Bateman plays a scared kid with major aplomb. It’s a serviceable horror film that will give genre fans a reasonably good time.