The Incredible Invasion (1971)

In the European village of Gudenberg in 1890, Prof. John Mayer and his assistant, Dr. Isabel Reed, have created a powerful ray machine. One of the rays is shot into outer space and attracts a flying saucer. The alien pilot decides that the ray poses too great of a threat to the universe and must be destroyed. Thomas, a sex maniac and serial killer, is possessed by an alien intelligence and infiltrates Mayer's household; later, Mayer himself is also possessed by an alien intelligence. Dr. Paul Rosten, a young chemist who is in love with Mayer's niece Laura, arrives. The alien-controlled "Mayer" and "Thomas" rig the ray gun to explode, killing all who know its secret. Thomas kills Reed, but before he can kill Laura, Mayer frees himself from the alien intelligence's control and Laura is saved. Mayer then realizes that the ray is too powerful for human use and destroys it. The flying saucer departs.