This is in fact not a film but an episode of the British television series Thriller from 1974 penned by the prolific Brian Clemens. Unfortunately it has had awful new American titles tacked on to the beginning which were clearly made several years later showing a murder taking place which then contradicts the body count mentioned by the policeman later on! But that aside, this is minor masterpiece. The plot is as miniscule as that of Halloween, the theme has been done to death in dreadful American tv movies, ie young secretary menaced by psychopath, but what really lifts this above similar yarns is its amazing sense of claustrophobia and highly strung performances. Julie Sommars does an admirable job of acting scared out of her wits, whilst Robert Lang is simply stunning as the silent killer who conveys the whole performance through movement and bizarre facial expressions. Clemens was clearly interested in breaking cliches; in a previous and less successful Thriller segment he replaced the old haunted house story with one about a haunted Rolls Royce; here he makes a modern office block after everyone has gone home into a chilling maze of danger and isolation, a "glass cage" as one character describes it. The situation is never stretched too far, the heroine is resourceful without seeming superhuman and the suspense reaches truly unbearable levels, particularly in one petrifying moment of fright near the end that takes one totally unawares, rather like a similar scene in "Wait Until Dark". As a viewer commented in the TVTimes after transmission, "this was bidding for some kind of award. I can't remember any other programme where I was too afraid even to light a cigarette for fear of missing a second of the non-stop action". There's also a superb scene early on when Ann returns home and first encounters the killer; they exchange glances as he walks past her and she feels uneasy. She then goes up the stairs and we see behind her his victim lying on the ground. Great stuff!