SEASON 1, EPISODE 105 - When a 5-year-old girl is kidnapped from her birthday party, Don and Terry lead the investigation but must rely on Charlie's help, as the girl's father, Ethan, is also a mathematician. Charlie realizes the kidnapper's motive when Ethan reveals that he is close to solving Riemann's Hypothesis, a difficult math problem. If solved, the solution could not only earn him with $1 million but could break the code for Internet security, unlocking the world's biggest financial secret. Neil Patrick Harris ("Doogie Howser, M.D.") guest stars at Ethan Burdick.