Only Joan (Phyllis Thaxter) knows the real reason she fainted at her engagement party and suddenly deserts her fiancé, Bob (Henry H. Daniels, Jr.). It’s the command of the sinister, vengeful “Karen,” a terrifying alternate personality raging inside Joan who will ultimately direct her to kill. Not even the psychiatrist (Edmund Gwenn) who discovers her fate can stop the tormenting voice that controls Joan’s every move – forcing her to run, to hide, to plot and to murder. Writer/director Arch Oboler, who adapted his successful radio play into this shocking thriller, keeps the suspense taut and the action filled with unexpected twists as Joan’s murder trial unfolds before a mesmerized courtroom.

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Radio legend and 3-D pioneer Arch Oboler brings his story, Alter Ego, to the screen in a low-budget yarn that benefits from a strong cast and direction. Joan Ellis (Phyllis Thaxter) hears a voice in her head (in flashbacks) shortly before she is to be married. She flees to another city and even takes up with another man to rid herself of the voice, but random words bring it back at unexpected moments. The voice ultimately tells her to kill her husband-to-be, and when a psychiatrist (Edmund Gwenn) determines on the eve of her execution that Joan is possessed by a split personality, a struggle ensues to see which one will survive. Oboler uses radio techniques and tense scripting to bring his thriller to visual life.