A neglected Hammer production ,this excellent thriller has enough sudden new developments to satisfy today's audience :no special effects (or so few) , no gore, no monsters, but there's no let up as the screenplay builds on suspense .The performances are over-the -top , which is fine with such a far-fetched story .The script uses old formulas ,but it works from start to finish ,with a surprise ending which is bound to catch all but the most astute unaware .