Boris Karloff and Béla Lugosi both star In The Invisible Ray (dir. Lambert Hillyer, 1936), which sees Dr Janos Rukh (Karloff) exposed to a mysterious element that turns him into a deadly luminescent maniac who devises a plot to bump off his personal and professional rivals.
Then, in Black Friday (dir. Arthur Lubin, 1940), Karloff is the amoral brain surgeon, Dr Sovac, who transplants part of a gangster’s brain into the body of his dying friend, creating a terrifying Jekyll-and-Hyde figure who starts murdering his former criminal associates (including, again, the wonderful Béla Lugosi).
Finally, the incomparable Charles Laughton plays the sadistic Sire Alain de Maletroit in The Strange Door (dir. Joseph Pevney, 1951). Maletroit has kept his own brother locked in a dungeon for twenty years, and now wants to ruin the life of his niece by forcing her to marry a violent rogue. His plan is upset when the “rogue” is unexpectedly noble-hearted and attempts to rescue the girl, aided by an abused servant (Karloff).
