Four girls have been murdered. As darkness falls, the young maid Helen Capel feels distinctly uneasy, as though she is being watched, as she walks through the woods alone.

News of yet another murder reaches the ears of the inhabitants, but this time close to their isolated family home on the Welsh border. Under the instruction of the Professor Warren, nobody may leave or enter the house that night. Just as the Professor finishes his announcement someone starts to knock loudly at the front door.... But is there really safety in numbers? And what happens when their numbers start to dwindle?

Ethel Lina White was born in Abergavenny, Wales, in 1876. She wrote from an early age, progressing from essays and poems, on to short stories, and eventually left her government job in the Ministry of Pensions to pursue a full time career in writing. White became one of the best known crime writers of the 1930s and '40s thanks to a string of successful titles, the most famous of which were Some Must Watch (filmed as The Spiral Staircase) and The Wheel Spins (filmed as The Lady Vanishes, including a 1938 production by Alfred Hitchcock).