A man fed through a wood chipper is identified as retired FBI Agent Bob Underhill. His wife reveals that he was very restless in his retirement, and they had since separated, but he had been looking into his old cases.
Sherlock soon finds a clue: the case file that is missing. The incident involved the kidnapping of a 10-year-old girl named Mina Davenport, who subsequently escaped her kidnapper 10 years later. Hunting for clues, the team questions the Davenports, who don't have anything to add.
While there, however, Sherlock realizes something important: Mina Davenport is an impostor, a supposition corroborated by a study of her ears in photographs. Sherlock confronts her, and she eventually confesses to being a con artist after Mina's trust fun, which is set to open in a couple weeks.
Fake Mina, a.k.a. Cassie, claims that Richard and Nancy Davenport killed the real Mina, and must have killed Agent Underhill, too, and she'll help them in exchange for the trust.
Sherlock agrees to her terms, but it's all just a pretence to gain access to the residence to find evidence against Cassie. They find traces of Underhill's blood in one of the cars, and Sherlock finally figures things out.
Cassie is arrested for killing Underhill, and Sherlock locates the real Mina, whose kidnapper is arrested, and she is reunited with her family.