After being taken to a South American prison island (actually, Alicante, Spain), where she will be imprisoned in a magnificent fortress called "The Castle of Death" for stabbing one of her rapists (as shown in a gorgeously stylized flashback), Marie (or Number 99, as she will be called from now on) soon learns to do things the stupid way, clashing with lead director Thelma Díaz (Mercedes McCambridge, trying to get out of a mid-career slump), when another newcomer (former Bond girl Luciana Paluzzi) suddenly goes into a state of jitters.
From the relatively respectable period of "eurotrash" emperor Jess Franco comes this shy precursor to the wave of WiP (Women in Prison) that would engulf exploitation cinema in the following decade, including, of course, many of Franco's own far more graphic reflections on the subject. Starting in the 1970s, the French distributor rescued the hardcore cuts and added them to the softcore version to produce the 98-minute version.