♪ it's one of those nights when you turn out the lights ♪ >> reporter: david cassidy was the ultimate teenage idol, known for his role as keith in "the partridge family," cassidy's fresh face and wide-eyed charm captured the hearts of millions of girls worldwide. >> you're taking auto shop? >> auto shop? me too. >>. >> reporter: "the partridge family" gave cassidy a national audience for his own music. ♪ i think i love you >> reporter: "i think i love you," the show's first single topped the billboard 900 in 1970 and sold over 5 million copies. >> i was always a musician. it was just fate and, you know, the way the stars align themselves. >> cassidy's wispy voice and wholesome persona broke out from the small screen and into sold-out arenas around the globe. his fan club at one time reportedly had more members than