The last of Universal's three Boris Karloff-Bela Lugosi teamings of the mid-1930s, The Invisible Ray is dominated by Karloff as Dr. Janos Rukh, the inventor of a laser-like heat ray. Despite the scoffing of his colleagues, Rukh intends to use the ray for the benefit of mankind, but first he requires a new element called "Radium X" to perfect his invention. Before long, he has embarked upon an expedition to Africa in search of a radium source, accompanied by his beautiful young wife Diane (Frances Drake), handsome young scientist Ronald Drake (Frank Lawton) and financiers Sir Francis and Lady Arabella Stevens (Walter Kingsford, Beulah Bondi). His system poisoned by increased exposure to radium, Rukh begins acting strangely, virtually forcing Diane into Ronald's arms. Apparently killed during the expedition, Rukh is actually alive, dementedly determined to use his "invisible" radium ray to do away with all his enemies. Soon he is able to kill with the mere touch of his hand, and this is how he disposes of his severest critic (and greatest supporter), humanitarian doctor Benet (Bela Lugosi). Rukh later tries to kill Diane as well but is unable to go through with it. The mad doctor meets his Waterloo when he is confronted by his own avenging-angel mother (Violet Kemble-Cooper). Not as lively as previous Karloff-Lugosi efforts, The Invisible Ray is nonetheless an effective melodrama.
by Hal Erickson synopsis