In 1990, thieves absconded with 13 masterpieces -- including works by Rembrandt and Vermeer -- from Boston's Isabella Stewart Gardner Museum, pulling off the greatest art heist in U.S. history. Rebecca Dreyfus's investigative documentary delves into this modern mystery, piecing together clues gleaned from archival documents, art critics, historians, collectors and informants (both credible and dubious) to shed light on the as-yet unsolved case.