Tonight's “Person of Interest” finds Reese in prison and unable to assist Finch in their quest. Finch hid a phone in his cell, and he is undercover as a math teacher to help the newest "Person of Interest," a teenager named Caleb Phipps. When Finch tries to clone his phone, he is unsuccessful; however, able to clone the rest of the kids in school. Caleb is a genius, who was hit by a train that killed his brother. He has survived, but is hiding his genius status.
Carter is attempting to help Reese get released but will need help. Finch does not want her to cross the line, but she admits that she crossed the line a long time ago. When Finch, a substitute teacher put a problem on the blackboard for a disinterested class, Caleb completed the problem and threw it in the trash on the way out. Finch asked him about it, and he asked why he is going through the garbage. During the class, he gives them an eye-opening explanation about what pi represents and what it means to each of them.
Fusco went to Caleb's house, supposedly looking for info about a neighbor, but it seems that the mother has a drinking problem. In another class, Caleb is hearing about internet hackers. A piece of paper he dropped when Finch broke up an argument he was having with other students revealed a code. He is writing code in longhand because he has no computer at home, and transferring it to a computer in school. Finch knows that it could be extraordinary. Finch asks Fusco to track the two guys Caleb was having the scuffle with; he knows they deal drugs.
Carter is all dressed up, looking like a high-class woman as she goes into a bar. She eyes up a man who smiles back at her. They leave together, and he chides her for asking about his background and medical history. She giggles and says that she is a practical girl and wants to know what she is getting into. He passes out in the car, and she takes a swab of his mouth.
Caleb tucks his drunken mother in her bed and goes out. Finch calls Fusco to let him know that he is on the move. The drug money has been followed by Fusco all night, and now it seems that the last transfer is a post office box. Lo and behold, the mystery drug dealer is Caleb.
During class, Caleb got an email to meet his supplier. Finch called Fusco to tip him off. After the class, the computer teacher sat at Caleb's station and copied a file to a thumb drive.
While Finch and Fusco follow Caleb, he finds the person he is looking for lying on the ground. Two people with guns drawn, kick a money bag at him. One asks why he would be dealing in his neighborhood. Finch and Fusco lay back and hear what goes on. The head guy with the gun tells him he must return all his cash that he took from his business by 8 p.m. the next night. Caleb tells them he will rob a bank.
The computer teacher is on the phone with someone and tells them they will find this interesting, as he looks over Caleb's code. Finch leaves Fusco to find out just what he has up his sleeve. He meets with an old college buddy in a restaurant and shows him the code. It is the Library of Congress, all on one thumb drive. Now they will need a new code. He needs an investment firm to get it off the ground. The buddy knows that he was not that great a coder in college. He makes him swear that nobody else is involved in this project; which the teacher does, all while Finch listens in the booth next to them? Now Finch knows why Caleb's name came up as the latest "Person of Interest." Will the teacher will try to kill him or the drug dealers?
Carter is still trying to break Reese out of Rikers Island. She breaks into the crime lab to switch the DNA sample that will acquit Reese.
Caleb has two appointments at 8 p.m. One is with the dealers and the other with the computer teacher. Both Fusco and Finch are keeping an eye on him. Fusco meets with the cop who changed the report when Caleb's brother died. The cop told Fusco how his brother died; they were playing a game, and Caleb blames himself; not the drug dealers in school.
Finch confronted the teacher, who told Finch that he was not stealing Caleb's work. They had a deal. Caleb wanted money to put into a trust account for his alcoholic mother. He told Finch that Caleb would soon be the richest 18-year-old in the country. However, Finch is worried that he will be dead before that can happen. Caleb plans to die at the same time as his brother; at the exact same age. Finch saved his life, and told him that he could no longer hide his genius, but be careful with his code.
As the episode is about to close and Reese is about to be released, the FBI agent in command orders him held. Because Carter was an interrogator in Iraq, she will be assigned to get the info from Reese.