Horace Manning, scientist, recluse and "closed book" even to his friends is found dead in his study at 4am, following a dinner in honor of his daughter's engagement. An ivory-handled carving knife rests between his shoulder blades as the house guests gather round to witness the awful crime. The telephone line has been sabotaged - a calculated murder has been committed.

Rewinding twelve hours, the events of the afternoon and evening unfold, revealing a multitude of clues and motives from a closed cast of suspects until the narrative reaches 4am again - then races on to its riveting conclusion at 4pm as the reader is led twice round the clock.