Robin Williams could improvise faster than any comedian alive. His mind moved at a speed that left audiences breathless and interviewers helpless. When that mind began to betray him — misdiagnosed as Parkinson's disease but actually Lewy body dementia — he lost the one thing that had always saved him.

This episode traces Williams from his lonely childhood through his rise in stand-up and film, his battles with addiction, and the neurological disease that was only identified after his death in 2014.

• He was posthumously diagnosed with Lewy body dementia, one of the most severe forms of the disease his doctors had ever seen
• He won the Academy Award for Good Will Hunting, delivering one of the most quoted monologues in film
• He struggled with cocaine and alcohol addiction throughout his career
• His improvisation ...