Oculus Karen Gillan, Brenton Thwaites, Katee Sackhoff. Directed by Mike Flanagan. Rated R. Opens Friday.

As horror-movie premises go, a haunted mirror has the potential to come off as pretty ridiculous. Director, co-writer and editor Mike Flanagan takes a completely straight-faced approach to Oculus, and his seriousness pays off with a creepy, well-acted take on some familiar horror ideas. Doctor Who’s Karen Gillan is entertainingly unhinged as Kaylie Russell, who blames that haunted mirror for her parents’ deaths. Now that her traumatized brother Tim (Brenton Thwaites) has been released from a mental institution, she enlists him to help her rid the world of the mirror for good. Flanagan intercuts their present-day efforts with flashbacks to their parents’ last days, and the line between past and present becomes blurrier as the movie goes on. The climax is a master class in cross-cutting, although the ultimate resolution is a bit of a letdown. Flanagan creates characters worth caring about and puts them through hell, which is the smartest way to make a horror movie, no matter what the premise.