Eve Gill (Jane Wyman), an aspiring actress studying at RADA, is interrupted in the middle of a rehearsal by her friend, actor Jonathan Cooper (Richard Todd). The frantic Jonathan explains that he is the secret lover of flamboyant stage actress/singer, Charlotte Inwood (Marlene Dietrich). He claims that Charlotte visited him after killing her husband in an argument. She was wearing a bloodstained dress and Jonathan agreed to go back to her house and obtain another one. He found the body of Mr. Inwood at the residence, took a spare dress from the wardrobe and then tried to simulate a burglary gone wrong, only to be caught in the act by Charlotte's theatre maid and dresser, Nellie Goode (Kay Walsh). He is now on the run, wanted by the police.
Eve has long had a crush on Jonathan, but accepts that he is not interested in her romantically. She takes him to hide in a house near the coast owned by her father, Commodore Gill (Alastair Sim). Persuaded to help, the Commodore then notices that the blood on Charlotte's dress has been smeared on deliberately, not by accident, and he and Eve suspect that Jonathan has been framed. He angrily rejects this, destroying the dress and thus the best evidence they have on Charlotte.
Eve decides to investigate for herself. Posing as a reporter, she bribes Nellie Goode to pretend that she is ill and cannot work for Charlotte for a while. Eve then utilises her acting skills to affect the false identity and accent of a Cockney maid, claiming to be Nellie's cousin, "Doris Tinsdale", and takes the temporary job of replacing "her cousin" in order to infiltrate Charlotte's household.
In the course of trying to clear Jonathan, Eve meets Detective Inspector Wilfred Smith (Michael Wilding). As Eve, she and Smith get quite friendly and she has to go to great lengths to cover the fact that she is also "Doris" the maid when Smith visits Charlotte to ask further questions. As Eve, she is not able to get much from him on the progress of the investigation.
Out to court Eve, Smith visits her and her mother at their home in London. They are joined by the Commodore who drops subtle hints to Eve that Jonathan has left their house by the sea. Meanwhile, in spite of the tragedies that are surrounding her, Charlotte continues to perform at her West End musical show. She is secretly visited by Jonathan who wants her to accompany him abroad and tells her that he still has the dress with the bloodstain. Charlotte makes it clear that she will not give up her career for a hunted fugitive in any case she is secretly having an affair with her manager Freddie Williams (Hector MacGregor).
Eve again helps Jonathan escape the police and he hides out at the Gill's London residence. He thanks Eve for her support, but she feels torn since she is starting to fall in love with Wilfred Smith.
When Nellie Goode uncovers the deception that is going on, Eve manages to buy her off with blackmail money. With time running out, she persuades Smith to accompany her to a garden party where Charlotte is singing on stage in a large tent. During the performance, Commodore Gill gets a little boy to take a doll wearing a dress stained with blood, Gill's own, up to the stage. The sight causes Charlotte to collapse and Williams summons "Doris" for assistance. Seeing Eve attending to Charlotte and the stained doll's dress leads Smith to put two and two together.
Smith confronts Eve and the Commodore over their "amateur meddling". They persuade him to set Charlotte up into making a confession. Once the theatre has closed for the evening, Eve comes out of character and confronts Charlotte near a hidden microphone Smith and his men listening in to the conversation from the loudspeakers. Charlotte admits her involvement in her husband's death (making her an accessory) but denies committing the murder itself, blaming Jonathan.
Eve then sees that Jonathan has been brought to the theatre by the police, Smith having guessed that he was hiding in their house. She falls into hysterics, enabling Jonathan to get away. At this point Smith reveals to the Commodore that Jonathan really did kill Mr. Inwood and that he has actually killed before, though he got off on a plea of self-defence.
Hiding below stage, Jonathan confesses to Eve that Charlotte goaded him into killing her husband but actually in order to make way for Freddie Williams. The story he told Eve when she agreed to hide him was all lies. He has a temper that makes him lash out when provoked. When he threatens to kill her as well, Eve escapes and in the confusion that follows Jonathan is decapitated by the stage's safety curtain.