Spoiled Simon Ashby (Reed) should count his blessings: he's wealthy, aristocratic and about to come into a large inheritance.

But when a man (Davion) shows up claiming to be Simon's brother Tony (thought dead by a suicide eight years before), suspicious doubts mount all around.

In actuality, Simon knows this imposter can't be Tony. How? Because he murdered Tony himself!

So who is this stranger and what does he want? Will Simon's fragile sister Eleanor (Scott) be able to survive the emotional stress?

And what role does Simon's stern aunt Harriet (Burrell) play in all this?

Following in the footsteps of Hammer's other post-PSYCHO thrillers, such as Scream of Fear (1961) and Maniac (1963), the plotline for Paranoiac is a solid one, and bolstered by a stalwart cast (Reed is exceptional here).

Indeed, fresh from his rather recent - and sympathetic - turn as the bedevilled beast in Hammer's exceptional Curse of the Werewolf (1961), Reed seems to delight in his twitchy psycho role here.

There are some terrific moments of chill-inducing suspense in Paranoiac too, most notably Simon's creepy organ-playing "performances" with "Tony" in the family chapel.

And that 'Paranoiac mask' is utterly spook-inducing, foreshadowing Alfred Sole's masked killer in the later Alice Sweet Alice (1977).

On the whole, underappreciated.