A curator at a museum, Lyle Endicott, is informed by his boss that he's being fired for incompetence. Right afterwards, he overhears a conversation between his boss and one of the museum's wealthy patrons, Martha Cheney. They are discussing her prized family heirloom, a vase that she refuses to sell. Lyle manipulates the same museum secretary who got him the job in the first place to forge a letter from his boss, saying what a great worker he is. The letter is sent to Martha Cheney, and Lyle manages to secure a job working in her home as a handyman and companion. He weasels his way in through charm and flattery, and soon as the old lady trusting him completely.
One day Lyle intercepts a letter to Martha Cheney, inquiring about purchasing the vase. He holds onto the letter and begins to devise a plan. First, he breaks one of Martha's tea cups and frames the maid, Bella, who professes her innocence. Although she's worked for Martha nearly 20 years, Martha believes Lyle over her loyal maid. Lyle convinces Martha to let Bella take a vacation, and in the meantime, a new maid is hired, who conspires with Lyle.
Lyle laments to Pamela back at the museum that he doesn't know where Martha has the vase hidden, but he's determined to find out. Meanwhile, Martha's trust for Lyle is eroding, and she grows weary of him, especially since he has been keeping her a virtual prisoner in her own home and giving her sedatives to make her judgment more hazy. On top of it all, Lyle has been feeding Martha lies to make her feel as though she were confused and not remembering events as they happened. Frustrated to the point of madness, Martha sneaks the new maid a letter in order to communicate with the outside world, since Lyle has disabled her telephone as well.
Martha tries to tell Lyle that their arrangement is no longer working out, but she is quickly reduced to impotent tears when he shows her the letter that the maid intercepted and gave to him instead. He shreds it in front of her.
One day, Lyle finally spots the vase, which he recognizes from a picture that he saw in the museum's archive encyclopedia. It is on a shelf in Martha's studio where she carves, but he can't take hold of it because she's there in the room. Meanwhile, he concocts a lie to Pamela, telling her that the vase is in a safe deposit. But Pamela has checked up on him when the passport office called her for a reference, and she knows that he's lying. As he's about to ditch her for good, reminding her that if she goes to the police she incriminates herself as well, the phone rings. It's Mr. Koether. Pamela hurriedly warns him to rush over to Mrs. Cheney's place, but Lyle strikes her across the face, sneering that it will be over an hour before Koether can make it there.
Lyle gets over to Martha's house and pushes open the door to the lab, only to find to his astonishment that the old woman has several shelves full of replica Cheney vases – each of them looking exactly alike.