The ancient and mysterious house of 'Mark's Priory' is the family seat of the Lebanon family. Lady Lebanon (Helen Haye) is desperate to have an heir to carry on the family name and has told her son, Lord William, (Marius Goring) that he must marry her niece, Isla Crane (Penelope Dudley-Ward). But Lord William has non intention of marrying, and Isla has fallen in love with a young architect who is renovating of Mark's Priory.

Lady Lebanon's desire to have the Lebanon name continue, along with her doctor's scheming intrigues, creates a crescendo of tension that only murder can release. But who is the homicidal maniac, and what sinister motives lurk beneath the servants' strange behaviour? As the police are called to investigate, the shadows of terror and death lurk in every corner of Mark's Priory.