The movie opens with a psychiatric evaluation of a young man named Tim (Brenton Thwaites) who saw the mirror drive both his parents crazy, ending in a bloody double murder for which he served time in an asylum. He's released on his 21st birthday, at which time his surviving sibling, Kaylie (Karen Gillan), insists that he revisit the mirror and destroy whatever entity resides within.
Psychiatrists, however, have convinced Tim that his memories of ghosts and haunted mirrors are coping devices to protect his mind from the awful truth of what happened.
So, when Kaylie unveils her plan to use high-tech surveillance and a remote-controlled Rube Goldberg contraption to destroy the mirror, he thinks she's the crazy one. The movie is meant to exist in a suspenseful place between these two possibilities.
Heightening that tension is the central gimmick here: The mirror's evil power rests in its ability to cause psychic delusions, so those in its vicinity are never sure that what they're seeing and feeling is real, and neither are we.
This scenario takes advantages of cinema's unique ability to suggest competing visual realities, all at once (the movie is directed by cult fave Mike Flanagan, of "Absentia").
Tim and Kaylie, for instance, are soon wandering in front of the mirror with their childhood selves (via cleverly integrated flashback) and delusion-driven versions of their contemporary selves.
Flanagan has ghoulish fun with the possibilities. Is Kaylie chewing a lightbulb, thinking it's an apple, or is that a delusion? Still, the movie's game of gotcha grows wearying, or at least it did to me (I'm in the minority).
Especially as the multiple realities all point to the same inevitable conclusion.
Stare into this particular mirror for any length of time, and it will tell you exactly how this movie is going to end.