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.