After U.S. airman Nathan Paet is shot to death in his garage while leaving for work, detectives can't find anything that would make him a target for murder until a witness comes forward to reveal the crucial missing details of Nathan's last day.

Kelsie Schelling disappears one night after leaving work, leading family and investigators on a four-year hunt to find the pregnant 21-year-old; cell phone records reveal what happened to Kelsie on her last day.

Investigators delve into forensic evidence from the scene and mobile data to solve the crime.