Keith Morrison recalls how a deadly high-speed chase in Montana led "Dateline" on a search to unravel a mystery unlike any he has told, one where a missionary turned into a monster and committed one of the state's most notorious crimes.

A 911 call reporting an apartment break-in holds the key to solving the mysterious murder of a young graduate student named Katie Sepich; her case contained many twists, and her family's grit still astonishes Josh Mankiewicz.