Prime Suspect is the fifth episode of the first season, and the fifth episode overall. It was released on February 18, 2005.
A genius' daughter is captured by two men looking to use his possible solution to a 150-year-old math equation for daylight robbery, making Charlie's input even more essential than usual.
Neil Patrick Harris
A birthday party is happening for a little girl. Her father, Ethan Burdick, goes inside partway through to continue on with his work. The party ends. As guests leave the mother goes to pay. A party clown leads the little girl, Emily Burdick, to a black van and they pull her in. Emily's mother, Becky, chases after them.
When Don inspects the father's office he sees complex math equations and calls in his brother, Charlie. Don notices Ethan on the phone just before he kicks the FBI out of the house.
Charlie introduces himself to Ethan who eventually recognises him. He had attended a lecture that Charlie gave that gave him insight to his work on the Riemann Hypothesis and he has finally found a solution after 15 years.
The FBI continues to work on finding Emily even though the Burdicks have shut them out. The Riemann Hypothesis is found to be a good motive for kidnapping for ransom. David goes to talk to people from various math journals to see if he contacted any. One - Steven Atwood - said that he did.
Terry recognises one of the clowns as Paul Ballard who had kidnapped at least two other people before this. He just got out of prison for tax evasion with no parole.
Don comes to the house where he and Charlie are introduced to a woman by Alan. They think the woman is Alan's date, but she is a realtor - Alan is selling the house. Charlie is upset. Don thinks it's a good idea.
Terry goes to the Burdick's home to ask about Ballard. Becky Burdick ends up telling her to go away.
The next morning Charlie starts to explain how numbers are used in encryption. The Riemann Hypothesis could be used to break through internet encryption. They conclude that this is why Emily was kidnapped. They identify Ballard's old cellmate as a hacker on parole. David finds out from Amita and Larry that his old cellmate, Carl Mittendorf, was the student of Dr Atwood.
Charlie tries to plead with Alan to not sell the house, but Alan doesn't budge.
Charlie goes to Ethan and convinces him to let him help with the math. Charlie finds a flaw in the math which will make it so that no algorithm that comes from it will not work, putting Emily's life in greater danger.
Ballard and Mittendorf sent the Burdicks a random tape. Emily looks to be ok, but just wants to come home.
Don comes up with a plan to give them a version of what they want to fool them into releasing Emily. Charlie works with Larry and Amita to come up with a passable algorithm. David goes to talk to Mittendorf's ex-girlfriend about his whereabouts and plans. He's going after the biggest financial secret.
At the Burdick's home, Charlie and Amita are helping Ethan to create the fake algorithm. They point Don in the direction of somebody using a super-computer. Another ransom tape is dropped off via messenger with drop-off instructions. Charlie suggests they create a fake door for the fake algorithm to open. There's only one problem, they don't know what door they're opening.
Don realises that they're going after information on interest rates and has a fake site created. Terry finds information to support the theory as they monitor the Federal Reserve website. They send the completed fake key to Ballard and his team. Mittendorf cracks into the fake site and the FBI are able to track down the house where the kidnappers are.
The FBI breaches and finds Emily in the basement with Ballard who uses her as a human shield against the agents. Terry has a shot through the basement window and Ballard lets her go. Don gets her back home to her parents.
Alan and Charlie finish dinner and Alan lets Charlie know that he's accepting a lucrative offer on the house. It turns out that Charlie is the person that made the offer on the house.
The issue of prime numbers is a key one in this case.
Ironically, given the theme of the episode and Ethan Burdick's research, none of the opening numbers (203,900; 90%; 56%; 48 hours) are prime numbers. The episode number (5), however, is.
The kidnappers go after confidential Federal Reserve data. When asked specifically what they are after the agent monitoring the break-in says that they are after the prime rate. While it ties into the episode title of Prime suspect and the episode's theme of prime numbers, the Prime Rate is NOT one of the rates that is set by the Fed.
[This appears on the beginning of the episode] 203,900 Child Kidnappings, 90% Parents Responsible, 56% Children Found Alive, 48 hours Critical Window