A mysterious package left on the doorstep holds literal and figurative keys to a preteen sleuth's shadowy past in this series closerDetective skills gained in their two previous cases prove useful to Colin and his partner and best friend, Nevaeh. Haddix pitches the pair into an investigation that, despite Colin's mother's resolute stonewalling, uncovers some shocking facts about his recently deceased and previously long-absent father. Fueled by a credible mix of anger at being apparently abandoned and a yearning just to know what happened and what his dad was like, Colin's inner odyssey makes up the emotional core of the story. But Haddix weaves in a substantial and increasingly suspenseful caper featuring clues found in documents and stored possessions, cold evidence trails to follow, startling revelations of both past misdeeds and unexpected family connections, and (to raise the stakes even more) a stash of money massive enough to put lives in danger. Some clues seem contrived, and the author repetitively makes use of characters asking themselves or others strings of questions. Still, events work out in ways that are both nonviolent and healing, as do certain issues that have formed a continuing theme through the trilogy involving the families of both young leads. Colin and Nevaeh read white, separate members of the supporting cast are Black and Latine, and one of Nevaeh's brothers is cued as neurodivergent.Tidy and upbeat.(Mystery. 8-12)