S1·E5 Prime Suspect

When a five-year-old girl is kidnapped from her birthday party, Don and Terry lead the investigation, but must rely on Charlie's help because the girl's father, Ethan, is also a mathematician. Charlie realizes the kidnapper's motive when Ethan reveals he is close to solving Riemann's Hypothesis, a difficult math problem. If solved, the solution could not only earn him $1 million, but could break the code for internet security and unlock the world's biggest financial secret.