Night falls in the capital, and a “London particular” pea-souper fog envelops the city. In Maida Vale, Rose and her family doctor Tedwards struggle through the dark after a man has telephoned from Rose’s house, claiming to have been attacked. By the time they arrive the victim, Raoul Vernet, is dead.

The news he brought from Switzerland for Rose’s mother has died with him. Arriving to the scene, Inspector Cockrill faces a fiendish case with seven suspects who could have murdered their guest – family members and friends with alibis muddled by the suffocating fog and motives wrapped in mystery. Now, the race is on to find the truth before the killer strikes again.