Many of the psychological thrillers released by Hammer in the '60s and '70s very much riffed on the same, or VERY similiar, material, and Paranoiac is no exception. However, most of these movies were also pretty damn good, and Paranoiac is also no exception in this regard.
The plot revolves around a family unit that has experienced great tragedy. Oliver Reed plays Simon Ashby, a young man happy to spend his days getting far too drunk and frittering away the family wealth, Janette Scott is Eleanor, his sensitive, and perhaps mentally unstable, sister, and there's an elderly aunt (played by Sheila Burrell) trying to provide what little guidance she can. You see, Simon and Eleanor both lost their parents some years ago. Even worse, they then lost their brother, Tony (Alexander Davion), when he committed suicide, unable to deal with his grief. Well, that's what they thought. The rug is pulled from under their feet when Tony reappears one day, leading to suspicion, tension and the potential to murder.
Director Freddie Francis and screenwriter Jimmy Sangster, who would work together on a few other Hammer movies after this one, do a fine job of setting up the characters as people already struggling to deal with some frayed nerves before then adding more and more reasons for everyone to get more agitated. The plot is, apparently, loosely based on the novel "Brat Farrar" by Josephine Tey, but the basic elements have all been done numerous times before, and the main difference comes from the presentation.
The proceedings are livened up by a decent cast, with Oliver Reed being the shining star. Oh, he may not always provide subtlety and nuance, but Reed has charm and charisma in abundance. He dominates any scene that he's in, and the film is all the better for it (as it's in line with his character anyway). Scott is just fine in a more restrained role, and Burrell is quite good at trying to care for the main characters while also being exasperated by them. Davion is charming, and does a convincing job as his character develops from the second to the third act.
With some nice design work, and some lovely cinematography by Arthur Grant, this is a solid little thriller that I recommend to any cinephiles who have somehow missed seeing it so far.