In the wake of a deadly virus, only a medical researcher, Dr. Neville (Smith), is left alive, in Francis Lawrence’s adaptation of novelist Richard Matheson’s last-man-on-earth sci-fi thriller. During the daylight, Neville cruises around a surreally empty Manhattan, now home to weeds and wildlife. (Abandoned buildings decorated for the holidays are a nice sardonic touch: Even for the last man on earth, the overly commercialized build-up to Christmas will never end.) Nighttime has him hunkered down against the infected — super-fast humanoid killers. Lawrence’s film is surprisingly bleak, and offers, to its credit, far less action, quipping and actor showboating than you’d expect from an A-list Hollywood production. Smith dials back his cheery charisma considerably for this role: His Neville is depressed, frightened and losing his mind in the twisted playground that is I’ll-take-Manhattan. But if Legend’s first two-thirds are tense and reflective, its conclusion feels considerably less organic, with a hastily appended religious twist and a final two minutes of plot any intelligent filmgoer should simply disregard.