If Peter Collinson's intention when writing and directing this film was to present the most bizarre characters imaginable, then he has succeeded admirably. If, however, he was trying to make a serious thriller with genuine excitement, realistic situations and a meaningful underlying moral subtext, then he has failed utterly.
The story has married estate agent Bruce Victor (Terence Morgan) and his secret lover Barbara Willason (Suzy Kendall) shacking up in a penthouse suite in an unfinished tower block. A pair of knife-wielding hoodlums turn up, posing as meter readers, and proceed to hold the adulterous lovers at knifepoint. Bruce is tied up and forced to look on as the lecherous intruders get Barbara well-and-truly drunk and then degrade her for their entertainment.
The film is based on a stage play, and it comes across - unsurprisingly - as a very stagy, talky affair. This is not necessarily a weakness (films like Sleuth, made five years after this, proved that stagy and talky films can actually be very good). However, The Penthouse is not only stagy and talky - it is very unpleasant too. The characters are awfully hard to like and their predicaments are extremely difficult to care about. Director Collinson frequently demonstrated a fascination with violence and aggression during his career, and this is a perfect vehicle for his favourite two themes. Collinson also had a fondness for stylistic flourishes in his movies, but here his outlandish camera angles and visual/aural tricks seem merely self-indulgent and meaningless. For the first twenty minutes, the film's surreal style is oddly enjoyable, but it pretty soon becomes wearisome. On the whole, The Penthouse is a failure and the fact that it is rarely-seen ought to be viewed as a blessing in disguise!