In his castle, the Czech Baron de Berghman (Henry Kolker) waits tensely for news of his wife giving birth. When she does, the event is not a happy one for him, as it raises the possibility of the family curse being revived - his wife has given birth to twins. Legend has it that this family line was created in the murder of an older brother by the younger and this will happen once more, but as the babies are twins, how could this come true now? The doctor points out that Gregor was born a minute ahead of the younger Anton, so the prophecy could still play out, and the Baron is not going to be persuaded otherwise. Time passes, and the Baron and his wife die, leaving the land to Gregor, but he grows up to be hated by the populace for his cruel ways. Now, ten years after the brothers (Boris Karloff) last saw each other, they arrange to meet again...
For some reason Boris Karloff never starred in a Hammer Horror, but with The Black Room you can imagine what it would have been like if he had, with its draughty castles, middle European Gothic chills and the spectre of death hanging over everyone, not mention the grumbling villagers ever-present: it's like the template for the British studio's future played out twenty-five years before. Scripted by Henry Myers and Arthur Strawn, who also devised the story, the film awards Karloff with one of his best roles, as this being the tale it is, one of the twins, Anton, is the thoroughly decent one, and the other, Gregor, is the thoroughly evil and hissable one.
Karloff offers a performance to relish as Gregor, alternating between glowering and grimly grinning as he exploits the villagers, but paranoid with it as just about everyone wants him dead. When Anton turns up with his big dog at the local tavern, of course the chatter stops and so does the music, as is traditional, but Gregor, unwilling to leave his castle, sends someone to go and fetch him; during the carriage ride, a shot is taken at Anton by a would-be assassin hoping to kill Gregor and the mild-mannered Anton is suitably shocked. But not as shocked as he will be when he finds out that his brother, who treats him with hospitality, has a penchant for murdering the local young women.
It's not all moody, as there is a pair of uninteresting lovers as part of the plot, Thea (Marian Marsh), the daughter of Gregor's right hand man Colonel Hassel (Thurston Hall), and her beau, the dashing Lieutenant Lussan (Robert Allen). Naturally, Gregor has his eye on Thea and works out a way to marry her which unfortunately involves bumping off Anton in the dreaded Black Room, which everyone thought had been walled up to prevent any prophecy coming true. Gregor lures Anton inside (the trick photography to render the two Karloffs in the same scene is excellent throughout) and pushes him to his death into the pit of bodies - now there's no way that the younger brother can kill the elder, or is there? As Gregor imitating Anton, the star is surprisingly subtle, lifting what could have been a run of the mill historical thriller to higher realms - he's also great fun to watch and the ending is inspired. Good dog!