New York Times bestselling author Margaret Peterson Haddix delivers the thrilling conclusion to her middle grade Mysteries of Trash and Treasure series as Colin and Nevaeh unravel their most personal mystery yet.

Colin has spent all summer solving mysteries with his friend Nevaeh. But they've only ever dealt with other people's mysteries--ones that are safe for Colin to think about. He's still stuck on the mystery surrounding his own father, who his mother refuses to talk about and he can't remember meeting.

Then one morning Colin finds a shoebox on his porch with a note on top: "Your father wanted you to have this." Inside the box is a key. This new clue makes Colin even more determined to find out the truth about his dad and why his parents split up when he was a baby. Colin and Nevaeh begin investigating Colin's father in a quest that takes them from eerie storage units to lock-lined bridges to, strangely, secrets in Nevaeh's family.

But the closer they get to connecting the clues, the more trouble awaits them. A serious accident leaves Nevaeh's family reeling--and Nevaeh racked with guilt. And digging into Colin's father's past may lead him and his mom into even more danger. Can Colin and Nevaeh solve the mystery before it's too late?