United by curiosity and a deep friendship, two neuroscientists from different parts of the world embark on a more-than-30-year quest to decipher the puzzle of why several generations of an extended family numbering some 6,000 people in Colombia, South America, have been struck with early-onset Alzheimer’s disease. The scientists' discovery that a rare genetic mutation in this population plays a role, has major implications for the understanding and possible treatment of this mysterious and deadly disease on a global scale.