The Mystery at Lilac Inn is the fourth book in the Nancy Drew series, published in 1930, where Nancy investigates the theft of heirloom diamonds from her friend Emily at the Lilac Inn, a case complicated by an imposter using Nancy's name and leading to dangerous encounters with criminals. The plot involves Nancy helping Emily plan her wedding, uncovering a jewel thief, and getting into perilous situations, including being tied up on a sinking boat, before ultimately solving the mystery and recovering the jewels