Kevin is a man tormented by dissociative identity disorder who harbors twenty-three distinct personalities, each with its own voice, gestures and intentions. Under the sway of his most dangerous facets, he kidnaps three teenage girls and locks them in a basement, among them the observant and reserved Casey, who senses that her only way out is to manipulate the different inhabitants of her captor. As his psychiatrist tries to understand him from a distance, a war for control rages inside Kevin ahead of the imminent emergence of an entity feared by all the others. M. Night Shyamalan delivers an unsettling psychological thriller, sustained by a chameleonic performance from James McAvoy.