Don't Open the Door is a 1945 mystery thriller novel by Anthony Gilbert, the pen name of British writer Lucy Beatrice Malleson.

[1] It is the fifteenth in her long-running series featuring the unscrupulous London solicitor Arthur Crook, one of the more unorthodox detectives of the Golden Age.

On a foggy night, nurse Nora Deane heads to the house of her latest patient, an invalid Adela Newstead.

Mrs Newstead seems terrified and once alone asks Nora to make contact with a man named Herbert without letting her husband know.

Nora then contacts Herbert and finds out he is the late woman's brother, who suspect there is foul play involved.