London 1958. Hidden deep beneath the city's streets lies the world of Miss Brickett's a secret detective agency. 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 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 uncharted tunnels that surround Miss Brickett's. Being discovered out of bounds means immediate dismissal but that is the least of Marion's problems. . .