In the novel, retired military police officer Jack Reacher becomes embroiled in a mystery involving a Vietnam War veteran who was reported missing in action, but who has resurfaced as a vicious loanshark with a secret he will murder to protect.

Reacher and Jodie follow Costello's trail, uncovering information on her father's last project: an investigation for the Hobie family on the whereabouts of their son Victor, a helicopter pilot reported missing in action during the Vietnam War.

They discover that the Hobies had been tricked into giving their life savings to a con man named Rutter, who poses as a fake military liaison to families of MIA soldiers.

After forcing Rutter to return the money he stole, Reacher and Jodie visit the National Personnel Records Center in St. Louis, which leads them to a forensic laboratory in Hawaii that identifies the remains of soldiers.

However, it becomes clear that Victor died in the crash, and that another soldier named Carl Allen assumed his identity in order to escape prosecution for fragging a superior officer.

Under his new identity, Allen amassed a fortune as an illicit "moneylender", before establishing himself as a legitimate businessman who offers high-interest loans to firms unable to borrow from banks.

Despite being aware that Reacher's investigation could expose his crimes, Allen decides to complete one final job: the takeover of a bankrupt multimillion-dollar company owned by Chester and Marilyn Stone.

At the hospital, however, a doctor discovers that, due to the arduous physical labor that was required for his job manually digging pools, his pectoral muscle was so thick the bullet did not make it past his rib cage.

[6] Tripwire received positive reviews from critics, with The Orlando Sentinel calling it "a thriller good to the last drop"[better source needed] and The Arizona Daily Star saying "Lee Child can write.