London, 1958: Elaborately disguised and hidden deep beneath the city's streets lies the world of Miss Brickett's, a secret detective agency. From traversing deceptive escape rooms full of baited traps and hidden dangers, to engineering almost magical mechanical gadgets, apprentice detectives at Miss Brickett's undergo rigorous training to equip them with the skills and knowledge they will need to solve the mysteries that confound London's police force. But nothing can prepare 23-year-old apprentice Marion Lane for what happens after the arrest of her friend and mentor, Frank, on suspicion of murder: he tasks Marion with clearing his name and saving his life. Her investigation will place Marion and her friends in great peril as they venture into the forbidden maze of uncharted tunnels that surround Miss Brickett's. Being discovered out of bounds means immediate dismissal, but that is the least of Marion's problems when she discovered that the tunnels contain more than just secrets...