Dr. Frank Peralta is found dead in his apartment, and a tense, twist-filled investigation begins. The case lands on the desk of Lt. Stevenson, Thomas Mitchell, who quickly takes note of two witnesses insisting that Peralta’s girlfriend Terry Collins was at the scene. Yet the trail soon hardens into a stubborn mystery: when Stevenson tracks down Terry, her alibi holds up under scrutiny, backed by several witnesses of her own. The plot thickens as it’s revealed that Terry has an identical twin sister, Ruth Collins, and the sisters share a job and routinely switch places for their own benefit, concealing which of them was at Peralta’s side that fateful night. The district attorney’s office, led by Charles Evans, finds itself unable to press charges because the twins refuse to confirm which one possesses the alibi, and the truth stays just out of reach.
Unable to accept what looks like a flawless crime, Lt. Stevenson reaches out to Dr. Scott Elliott, Lew Ayres, a specialist in twin studies, hoping to understand the bizarre dynamic from a psychological angle. Although Scott doesn’t yet know which sister is which, he agrees to study them individually as part of his research, using a careful front: he asks Terry and Ruth if they will participate, ostensibly to aid his scholarly work. Ruth fears the arrangement might expose Terry’s guilt, while Terry—attracted to Scott—insists they can keep the secret for the sake of continuing their unique companionship with him. She comforts Ruth, reminding her that she herself was only at Peralta’s apartment that night and did not kill him.
Through Scott’s tests and extended time with the twins, a startling profile emerges. Ruth is shown as kind and loving, the steadier, more gentle sister. Terry, in contrast, is exceptionally bright and, disturbingly, capable of calculating cruelty; she has manipulated Ruth for years and now harbors a fierce envy of the attention Ruth receives. In Scott’s terms, Terry’s intellect is matched by a dangerous instability, and her jealousy becomes a fuel for the plan to keep Ruth sidelined. As the psychologist’s findings circulate, Stevenson is urged to alert Ruth to the danger, and the path forward grows increasingly perilous.
That night, Scott arranges to meet Ruth at his apartment to convey what he has learned. Yet Terry intercepts the message, leaving Ruth alone in their shared flat and placing a music box in a hidden spot to trick the other sister into thinking she’s sliding into a false madness that twins sometimes experience. Terry then heads to meet Scott, and the two exchange a tense, revealing dialogue: Scott explains his discovery about the sisters’ relationship and Terry’s deep-seated rivalry with the innocent Ruth, all while he gradually realizes he’s not talking to Ruth at all.
A crucial turn comes when Stevenson, on a hunch, visits the twins’ apartment and pretends there has been a fatal development. He frames the moment as Ruth’s supposed death, prompting Terry to react with a convincing display of despair. Scott, who believes he has laid bare the truth, continues to plead with Ruth through the door, but a twist soon makes the situation clear: Ruth is very much alive, and the genuine Ruth’s presence shatters Terry’s constructed narrative. In a tense revelation, Stevenson’s staged phone call to Ruth is exposed, and Terry is finally cornered. The trap, designed to peel back Terry’s carefully maintained alibi, works as intended, and Terry Collins is arrested.
In the aftermath, Ruth’s innocence stands, Scott’s cautious but groundbreaking analysis has proven valuable, and the two sisters’ lives are left to unfold apart from the dangerous lie that nearly consumed them. The case closes with Ruth and Scott free to pursue their future, while the truth about the twins’ intricate relationship—and the perilous balance of love, jealousy, and intellect that fueled the crime—remains firmly in the record.
