"Penny Nichols and the Mystery of the Lost Key" by Joan Clark is a mystery novel first published in the mid-20th century. The story revolves around the protagonist Penelope "Penny" Nichols, a clever and adventurous young girl who finds herself entwined in a web of intrigue while assisting a new acquaintance, Rosanna Winters. Initially, Penny's life takes a dramatic turn after witnessing a theft in a department store, which introduces her to Rosanna, who has recently inherited an estate—leading Penny on a quest filled with potential dangers and hidden treasures. At the start of the story, Penny and her friend Susan are rushing to a jewelry counter just before the store closes, where Penny witnesses a man quickly stealing a valuable diamond ring. As she raises the alarm, she becomes involved with Rosanna, who is distressed after narrowly losing a significant letter and key that explain her inheritance from a deceased uncle. The opening chapters explore Penny's resourceful nature as she offers to drive Rosanna to meet a lawyer regarding her inheritance, creating a backdrop ripe for mystery as they discover that many others are also vying for the estate, each presenting their own claims and secrets. The mix of theft, inheritance, and personal drama sets the stage for an engaging mystery with a young detective at its heart. (This is an automatically generated summary.)