Some 30 million people have sent their DNA to be analyzed by companies like 23andMe and AncestryDNA, hoping to obtain clues to family origins and forecasts of their future health. Some users have found family members and discovered lurking genetic risks. But what happens once the sample is in the hands of testing companies? What are they looking at and how accurate are their results? NOVA explores the power of this information and the unintended consequences that can arise from sharing our data with these rapidly growing online databases. DNA results that report on health risks can be misleading, and the discovery of intimate family secrets can tear relationships apart. Meanwhile, law enforcement is increasingly turning to the DNA-sharing website GEDmatch as an extraordinarily powerful tool for cracking cold cases, as demonstrated by the 2018 arrest of California’s notorious Golden State Killer after 42 years at large. What is the peril and promise of consumer DNA testing?