More than 700,000 people are reported missing in North America every year. In many cases involving kids, AMBER Alerts produce a coordinated response by nationwide media and emergency services. But when someone 18 or older is considered a possible runaway, investigations often lack the same urgency, and family members are left to pick up the slack. "Last Seen Alive" documents the journeys of people chasing leads, interviewing potential witnesses, and traveling the country in search of relatives. Episodes follow the cases of two people, beginning at the moment they were last seen. The families -- desperate and with everything to lose -- get help from professional investigators who have had success locating people. Some stories have heart-warming conclusions; some end tragically; others still remain open.