"The Invisible Ray" is part science fiction and part horror. It was also the third of seven Karloff/Lugosi features. In this entry the dominant role goes to Boris Karloff.

Through specially designed astronomical equipment, Dr. Janos Rukh (Karloff) demonstrates to colleagues Dr. Felix Benet (Bela Lugosi), Sir Frances Stevens (Walter Kingsford), his wife Lady Stevens (Beulah Bondi) and their nephew Ronald Drake (Frank Lawton), that a meteorite containing a powerful element, landed on the African continent many millions of years in the past. Rukh impresses his guests who invite him to accompany them on an expedition to Africa to find the mysterious element.

Rukh goes off on his own and discovers the place where the meteorite landed and the element which he names "Radium X". Due to Rukh's long absences, his comely young wife Diana (Frances Drake) becomes attracted to Drake and the two fall in love. Meanwhile Rukh becomes contaminated by Radium X to the point that anyone he touches will die instantly. The contamination causes his skin to emit a bright glow in the dark.

Rukh goes to Benet for help. Benet devises an antidote which if taken on a daily basis, will provide temporary immunity to the element. Unfortuneatly, the deadly element also affects Rukh's brain, slowly turning him into a vindictive murderer.

While Rukh is continuing his work, Sir Frances takes news of the discovery back to France. Diana and Ronald accompany him. When Benet informs him of this action, Rukh accuses the party of betraying him and his discovery and secretly plans his revenge. When he returns to France he learns of the healing power of Radium X as he cures his mother's (Violet Kemble Cooper) blindness and of Benet's work in curing the maladies of his patients.

But Rukh's madness intensifies. First he murders an innocent man whom is identified as Rukh. Upon hearing of Rukh's apparent demise, Diana and Ronald marry. This angers Rukh and he begins to exact his revenge on the five other members of the expedition. One night he is lured into a trap set by Benet and...........

Karloff is excellent in the lead role moving from a happily married ambitious scientist to a raving maniac. Lugosi has a straight role for once and does what he can with the limited part.

At 79 minutes this film was the second longest of the six Karloff/Lugosi collaborations. Karloff's film all the way.

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