"Four Hundred Thousand" is a novel by M. J. Arlidge, part of the Helen Grace series. The story follows Detective Helen Grace as she investigates a series of mysterious disappearances in her city. The narrative combines elements of crime, suspense, and psychological thriller, keeping readers engaged with its twists and turns. The book explores themes of morality, justice, and the impact of past choices on the present. As Helen delves deeper into the case, she uncovers dark secrets that challenge her understanding of right and wrong, ultimately leading to a gripping conclusion that tests her resolve.