London, 1960. Marion Lane, a 25-year-old apprentice detective at the elusive Miss Brickett’s Investigations & Inquiries, is busier than ever and determined to prove herself worthy of an official Inquirer badge. But when her close friend's girlfriend, Darcy, is targeted by a dangerous gang leader and seeks out the Inquirers’ assistance, Marion cannot help but get involved. Just when Marion thinks she has the situation under control, Darcy disappears and the agency receives a package containing a dead raven. Everyone is puzzled by what the threat could mean, except for Marion. She recognizes it as the same calling card left on her mother's doorstep just before she died.

It’s 1959, and a new killer haunts the streets of London, having baffled Scotland Yard. The newspapers call him The Florist because of the rose he brands on his victims. The police have turned yet again to the Inquirers at Miss Brickett’s for assistance, and second-year Marion Lane is assigned the case. But she’s already dealing with a mystery of her own, having received an unsigned letter warning her that one of the three new recruits should not be trusted. She dismisses the letter at first, focusing on The Florist case, but her informer seems to be one step ahead, predicting what will happen before it does. But when a fellow second-year Inquirer is murdered, Marion takes matters into her own hands.

A mysterious group of detectives recruited for Miss Brickett’s Investigations & Inquiries use their cunning and gadgets to solve crimes that have stumped Scotland Yard. Late one night in April 1958, a filing assistant for Miss Brickett’s receives a letter warning her that a heinous act is about to occur. She goes to investigate, but at the stroke of midnight, she is murdered by a killer she can’t see --- her death the only sign she wasn’t alone. Marion Lane, a first-year Inquirer-in-training, finds herself being drawn ever deeper into the investigation. When her colleague is framed for the crime, she must sort through the hidden alliances at Miss Brickett’s and secrets dating back to WWII to uncover the real killer.