In a darkened London cinema, a man slumps in his seat. At first, the other patrons think he is only asleep, but then, to their horror, they discover that the man is dead, stabbed while they had been watching the film.

When Scotland Yard's Inspector Reynolds investigates he is surprised to find that the victim, a notorious blackmailer, is the same man who earlier that day had reported receiving a threatening letter bearing the single word "TO-NIGHT." With his only clue a witness's statement that a young woman entered the theatre with one man and left with another, Reynolds must try to solve the mystery of the Tragedy in the Dark.