[1] The story is told as a flashback: It is late at night, and Dr. Bergson (Edmund Gwenn) is dictating case notes to his secretary.

On the night of a lovely party celebrating her engagement to Bob Arnold (Harry H. Daniels Jr.), a demure young woman named Joan Ellis (Phyllis Thaxter) at last succumbs to the voice in her head (voiced by Audrey Totter, unbilled) and flees her Midwestern home to New York City, leaving behind a loving family and circle of friends who are shocked, bewildered and frightened for her.

Going by the name Joan Smith, she builds a new life in the city and falls in love with Eric Russell (Horace McNally), an attorney.

He is thrilled, but while he is helping her pack, Karen takes control of Joan's right hand and compels her to pick up a pair of scissors and stab him in the back.

She remains silent, and Eric uses this silence to convince the jury that there is more to the story and, whatever the truth may be, she is innocent of murder.

When I die, she dies.” This sets Dr. Bergson thinking, and, with the execution imminent, he persuades the governor (Minor Watson) to listen to his theory—that here are two women living in one body, one guilty and one innocent.

The use of sound effects, editing, stylized dialogue, and heightened, punchy music all reveal the radio influence.

Finally, Oboler gave Thaxter's evil personality its own voice, well supplied by an unbilled Audrey Totter, a film noir fixture (Lady in the Lake (1947), Tension, (1950)) whose career began in radio in 1939.”[3] When the film was first released, the staff at Variety liked it and wrote "Produced on a low budget, with a sterling cast of actors' actors, this picture just oozes with class because of the excellent adaptation and direction it has been given by radio's Arch Oboler, author of the story "Alter Ego", on which the film is based.