An ambitious housemaid learns her employer murdered his wife.

In turn of the century London, after the unexpected death of his wife, Stephen Lowry finds himself wealthy and alone. Scullery maid Lily Watkins, who is despised as incompetent and flirtatious by stern cook Mrs. Park, secretly learns that Stephen poisoned his wife. When Stephen summons Lily to inquire about his wife's jewels, she informs him that Mrs. Lowry gave them to her. Stephen accuses Lily of lying, but when she reveals she knows about the poisoning, he allows her to keep the jewels and promotes her to the position of housekeeper. Some time later, friend and business confidant Alfred Travers offers Stephen a partnership in his business and implies that the romantic relationship between his daughter Elizabeth and young attorney David MacDonald is not settled. Upon returning home, Stephen is outraged to find Lily trying on his wife's clothing and wearing her perfume, but nevertheless begins an affair with her. A few days later, David asks Beth if she is in love with Stephen and when she admits that she is, he cautions her about getting involved with him. David then fabricates a reason to visit Stephen at home, but when he mentions Beth, Stephen grows indignant, declaring that he is still in mourning. Mrs. Park and butler Grimes interrupt the meeting to protest being fired by Lily, and David notices that Lily has on the same broach that Mrs. Lowry is wearing in a portrait. Later, Stephen angrily warns Lily not to take such foolish risks, but Lily assures him she can take care of him without other servants or Beth. When Stephen suggests that she might better her situation by moving to Canada or America, Lily refuses, claiming that she will never leave him. Disturbed by Lily's increasing hold upon him, Stephen impulsively follows her when she leaves to deliver a letter and, stalking her through the heavy fog, beats her to death with his heavy walking stick. Hurrying away through the fog, Stephen grapples with two men leaving a pub and loses the walking stick, then hides in a tree as the body is discovered. When the crowd finally disperses, Stephen makes his way home, only to be shocked when Lily returns safely moments later. She remarks about the commotion in the streets over a murder and while hanging up Stephen's cape, finds it covered with blood and realizes his intentions. The police arrive to inform Stephen of the murder and reveal the victim as constable Burke's wife, but Lily covers for Stephen's whereabouts. The following day, Stephen is questioned by the police and when identified by the two men from the pub, arrested. Beth pleads with David to represent Stephen. At the trial, David casts doubt on the men from the pub by suggesting they were too drunk at the time to accurately identify Stephen. Lily testifies to having lost Stephen's walking stick weeks before and confirms he remained home the night of the assault. Stephen is acquitted based largely on Lily's unflappable testimony. Back at home, Stephen thanks Lily and confides that although he feels no guilt over poisoning his wealthy wife, he feels badly about the death of Katie Burke. When Stephen asks Lily if she is afraid of living with a murderer, she reveals that she has written a revelatory letter to her sister, Rose Moresby, but the note is to be opened only if something happens to her. Shortly after the trial, Stephen takes up his new post with Alfred's company and over the next few months, begins seeing Beth regularly. Lily is furious when Stephen announces his engagement to Beth, but he placates her by explaining that as Alfred's prospective son-in-law he will have greater access to the company's money, which they can then use to flee to America and marry. When Stephen mentions the letter in Rose's possession, Lily assures him that she will write and ask her to burn it. When Rose attempts to burn Lily's letter, however, her husband Herbert questions her and later secretly retrieves it from the fireplace. Over the next few days, in a plan meant to frame Lily, Stephen begins poisoning himself slowly and grows sick, bringing the doctor on several visits. Meanwhile, David visits Alfred in order to try to convince him of Stephen's involvement in Mrs. Lowry's murder, but Alfred refuses to believe him. Upon leaving the Travers' office, David is mistaken for Stephen by Herbert, who later shows David the charred letter and demands money for it. David takes Herbert to Alfred, who, along with Beth is upset about the letter's implications. At the same time, Stephen has a relapse and pleads with Lily to get the doctor as quickly as possible. In her absence, Stephen doses himself with an extra amount of poison, believing that Lily will return with the doctor in minutes. The police pick up Lily and bring her to Alfred's, where she submits to a writing sample, which she purposely alters. Terrified when Lily does not return, Stephen manages to call constable Burke for help. Before Lily hurries away from Alfred's, a constable gets her hurried signature, which matches the writing on the letter. Upon arriving back at home, Lily finds the doctor and Burke, who, having discovered the vial and jewels in her room, accuse Lily of murdering Mrs. Lowry and attempting to kill Stephen. Realizing Stephen's plan, Lily demands that he confess, but the dying Stephen only mutters that Lily threw off his timing. Faced with certain arrest and conviction, Lily laments that she was not Stephen's victim in the fog.