Interesting but hardly original drama with sci-fi leanings though not quite the "horror"/"chiller" described by the ads! involving the brain-washing of violence-prone subjects by the system (which must have seemed particularly trenchant at the time of the Vietnam war).
At this juncture, however, the movie feels quite dated if reasonably intelligent and compelling nonetheless. Being also relentlessly talky (not surprising, given its stage origins) and low-key in nature, there's a conspicuous lack of cinematic inventiveness which doesn't really allow for a sensible comparison with Stanley Kubrick's stylized treatment of the same theme in A CLOCKWORK ORANGE (1971)! Still, it has some undeniably powerful moments and the small cast is impressive: Christopher Walken (relatively inexperienced for this type of demanding role, but quite good in his Method approach to it); Joss Ackland (as the requisite mad scientist); Ralph Meeker (as the equally inevitable, and callous, military overseer); and Ronny Cox (as a fellow inmate of Walken's who, after much soul-searching, willingly submits to the dehumanizing experiment).
Incidentally, the play was filmed under its original title THE HAPPINESS CAGE but this got changed (in case it was mistaken for an ode to hippiedom) first to the sci-fi friendly and, in retrospect, more appropriate THE MIND SNATCHERS and eventually to the horror-oriented (and, consequently, wholly misleading) THE DEMON WITHIN!
