The Machine gives the Social Security number of a girl that has been dead for two years. Reese and Finch try to track down an apparent ghost while Finch remembers his partner's objections when he first created the Machine.
Reese and Finch try to help an ex-Marine who is working with a gang of other ex-soldiers who rob banks to cover their debts. Meanwhile, Carter comes to the realization that Reese isn't the cold-blooded killer she initially believed.
Dr. Megan Tillman, a workaholic ER physician, comes up on the Machine. While Reese and Finch watch her, Fusco has to deal with a drug cartel that wants their drug money back.
When a judge's son is kidnapped, Reese and Finch try to help him. However, he isn't interested in putting his son in further danger by accepting their help.
A Cold War assassin goes after the members of his former team, and Reese and Finch try to stop him. Meanwhile, Fusco attempts to convince Carter that her shooter can do some good.
The Machine gives Reese and Finch Carter's number, and they are forced to deal with a number of suspects when they realize that crimelord Carl Elias has hired someone connected to her to kill her.
The Machine gives Reese and Finch four new numbers, and they soon discover that one of the Persons of Interest is already dead. Meanwhile, IA investigates Carter for her involvement with the mystery man, and the CIA tells her that they want Reese back.
While Reese, Carter, and Fusco protect a young teenager out for revenge on the gangers who killed his brother, Finch worries that his dead partner's son Will will track down the woman who knows about the creation of the Machine.
Reese and Finch discover that their newest Number is a Wall Street prop trader who may be in involved in a multi-million dollar scam that will rob thousands of investors of their life savings.
Reese and Finch must deal with a new Number who has very little electronic footprint. However, they soon discover that their "client" is actually two people with the same name, SSN, and bank accounts... and one of them is in great danger.
Finch tries to keep Reese off of a case by giving him his birthday off, but Reese soon learns about it anyway and realizes it related to a personal incident in his life.
Root explains to the captive Finch what she plans to do for the Machine. Meanwhile, Reese tries to track down his partner but discovers that the Machine is less than cooperative.
Carter and Reese travel to Texas to unravel a 21-year-old missing child case and get a line on Root... the mysterious woman who has abducted Finch and Weeks, and plans to find the Machine no matter what it takes.
Reese and Finch discover that the man they've been assigned is a mob enforcer ordered to kill the wife of a dead mobster. However, they soon discover that the enforcer is in love with the woman, and have to decide rather to protect him as well or let him die.
The Machine directs Finch and Reese first to Leon Tao, who has gotten in trouble again, and then to a doctor who is being forced to kill a patient to save her wife.
An unhappy couple hire hitmen to dispose of each other, and both of their Numbers come up. Meanwhile, love is in the air for Fusco and Carter, while Finch reminisces about his past with Grace.
A Cuban defector comes up as the next Number and Reese and Finch try to pinpoint who might be after him. Meanwhile, Carter tangles with the Secret Service while HR tries to make amends with Elias and sends Fusco to negotiate.
While the FBI close in on Reese, he fights to save a young couple who are out to take down a crooked loan officer who conned military veterans out of their money. Meanwhile, Carter gets a job offer and Fusco investigates her new boyfriend.
While the FBI try to confirm that Reese is the man they've been seeking, Finch and Fusco take on a new Number: a high school student with knowledge of computers.
At Donnelly's request, Carter interrogates the captive Reese in an attempt to confirm that he is the "man in the suit." Meanwhile, Reese tries to dodge an assassin sent into prison to kill him while Fusco protects a super-model whose Number is up.
Reese's former partner and lover, Kara Stanton, abducts him and forces him to participate in her plan to break into a DoD facility and steal a computer virus.
Reese and Finch try to help a CTU operative, Sam Shaw, who receives her targets' Social Security Numbers through the government. When she learns too much, the government wants her dead... and when Reese and Finch find her, Sam refuses their help.
The Machine sends Finch and Reese six Numbers and they soon discover that they belong to six missing persons... and the sixth one disappeared on an island in the middle of a heavy storm. Meanwhile, Carter questions Cal's trustworthiness after Moss warns her about the detective.
The new Number is an old man, Lou Mitchell, who blows thousands of dollars each week while gambling at a New Jersey casino. While Finch and Reese try to work out how Lou does it, and why he's in danger, Carter helps a fellow detective framed by HR.
While Finch tries to protect an estate investigator whose Number has gone up, Reese searches for a missing Sam... unaware that Root has convinced Sam to help with a mission for The Machine. Meanwhile, Carter teaches Laskey about the reality of how HR operates.
The Machine sends the team the Number of a conman who claims to be a hypnotherapist and uses the information he obtains to access his patients' bank accounts. However, it turns out that one of the patients is a money launderer for HR... and they're not too happy to learn that someone is ripping them off.
The Machine gives Finch thirty-eight Numbers, and he realizes that someone is planning to set off a war between HR and their Russian Mob allies. Meanwhile, Carter continues her own investigation into HR and remembers her past with her ex-husband.
As Carter and Reese try to turn the head of HR over to the Feds, they discover that the organization has set up roadblocks and cut off every means of access. Finch tries to rally the the troops to help them, and is forced to turn to an unlikely ally who demands a high price for her assistance.
The hunt is on for Simmons after he kills one of the team. Finch and the others try to stop Reese from killing the crooked cop on the run, but soon realize that he's not the only one who wants Simmons dead.
Finch and Sam try to recover the Samaritan drives from where Arthur hid them, but Vigilance and Control's team move in on them, cutting off their escape. Meanwhile, Control interrogates Root and gets unexpected answers, while Fusco tries to convince Reese to come home.
Reese tries to leave the country after Carter's death, but The Machine manipulates him into saving a Number... who may also be a "relevant."
The team's newest Number is a 911 operator, Sandra. Finch signs on at her call center as a trainee and soon learns that a mysterious caller plans to have Sandra erase valuable files... and will kill a young hostage if she doesn't.
In 2010, Finch and his enforcer help their newest Number, a programmer who two CIA agents want dead.
While Decima gathers its resources to build a new Machine, the team receive the Number of an electrical engineer who is caught up in UN intrigue trying to help an Iraqi translator receive asylum in the U.S.
Finch sends Reese and Sam to a high school reunion to protect their new Number, while he travels to DC with Fusco to look more closely into the death of their last Number at the hands of Vigilance.
Samaritan goes online for 24 hours, monitoring New York City, and Greer uses the opportunity to find Reese, Sam, and Root. At the same time, he has his people try to locate anyone with a connection to the missing Finch... and comes up with Grace Hendricks.
The Machine predicts a major upheaval coming and sends Root to work with Reese and Sam to get it the intel it needs. Meanwhile, Greer talks with Finch, and Vigilance makes its move as Collier's hatred for the government is explored.
While Reese and Hersh try to track down Finch, who is on trial by Vigilance, Sam helps Root sabotage Samaritan. Meanwhile, Finch has no choice but to confess that he created the Machine and defend his actions.
The Machine finally sends a Number to the undercover Reese and Sam, and Reese tries to convince Finch to help them. When he won't, Reese is forced to go it alone to save two lives.
Reese turns to a reluctant Finch for help with their new Number, a gifted mathematics student who is trying to win a bizarre urban game with a sinister ulterior motive. Meanwhile, Sam pursues her second career as a street thief.
Fusco goes undercover as a man desperate to get a date, so he can get close to the new Number, a professional wingman. Meanwhile, Reese tries to impress the new captain, while Root takes Finch on a scavenger hunt on behalf of the Machine.
When a young brother and sister witness a gangland shooting, they take a half million in drug money and go on the run. Reese goes looking for the team's two new Numbers and finds the Brotherhood after them. Meanwhile, Finch turns to Elias for help.
While Reese is forced to undergo therapy for his violent tendencies, Sam, Root, and Finch try to help a pollster who has run afoul of Samaritan.
An unassuming insurance agent becomes a private detective to investigate the apparent suicide of a co-worker's brother, but soon gets in over his head. Meanwhile, Finch travels to Hong Kong to present a paper.
Finch sends Sam in to infiltrate a gang of thieves whose leader is the team's new Number. Meanwhile, Root helps Finch undermine Samaritan's newest scheme.
Reese volunteers to teach at the police academy to keep an eye on the team's new Number, a recruit who is spying on her fellow classmates. Meanwhile, Mini launches his plan to get the information he needs to bring Elias down.
Samaritan finally offers the Machine a face-to-face meeting... by stopping crime in New York City for a day to demonstrate its power. It then unleashes chaos to prove that humanity is flawed. Meanwhile, Reese and Finch struggle to keep up with the escalating Numbers, and Sam chafes at being concealed from Samaritan.
Control discovers that the new Research division of the ISA is shutting her out from valuable information. When Decima forces her to back down, she continues the investigation on her own... and Reese and Root capture her in a bid to find their teammate Sam.
Reese and Root investigate a small town in upstate New York that holds the key to Sam's location... and whether she's dead or alive. Meanwhile, Fusco investigates a new Number and runs into former Number Dani Silva.
The Machine arranges for Finch to sit on the jury of a man on trial for the murder of his wife. He discovers that someone is trying to fix the trial, and Zoe volunteers her services to help find out who the fixer is. Meanwhile, Reese meets voluntarily with Iris for more therapy sessions.
The team's new Number is a therapist who has a unique way of bringing closure to his patients. Meanwhile, in the past, Finch ignores the Machine's warnings and tries to kill Corwin for her part in Nathan's death.
A antivirus CEO's life is destroyed when his computers are hacked and his information is exposed to the world. As he tries to put his life back together, Samaritan launches a campaign to eliminate him for good.
As Root and Finch follow a lead to Sam, Reese and Fusco try to find Elias and protect him from Dominic before it's too late. Meanwhile, Control questions one of Samaritan's handlers and learns about The Correction.
The Machine ends up malfunctioning during a reboot and gives Reese and Fusco the Numbers of dozens of people not involved in any crimes. At the same time, it flags Finch, Root, and Reese as threats, and shuts Finch and Root out of the system.
Reese's cover could be blown when the team's latest Number turns out to have ties to Reese's old boss in the CIA. Meanwhile, Finch and Root turn up Samaritan's newest scheme.
Sameen escapes from Decima's secret hospital, but she soon realizes that nowhere is truly free for her except for one place.
The team's newest Number is an analyst, Ethan, handling the a new ShotSeeker program that identifies gunshots anywhere in the city. They soon discover that Samaritan is involved. Meanwhile, Elias' friend tries to learn the truth about what happened to the crime boss.
The Machine sends Root to protect a radio host who has intercepted secret messages from Samaritan to its operatives... and to contact the captive Sameen.
The team's newest Number drops dead in a hospital, and Reese and Finch soon realize that he was deliberately infected with a superflu. Meanwhile, Fusco works with Elias to track down one of Samaritan's unwitting operatives, while Sameen finds herself in a prison in South Africa.
With one teammate missing and another dead, the three remaining teammates must travel to DC to save their new Number: the President of the United States.
Finch prepares to make his final move against Samaritan, even though it will cost the Machine its existence. Meanwhile, Fusco receives a visit from an old friend, and Reese and Sameen race to infiltrate the NSA.
Finch, Reese, Fusco and Shaw take part in one last mission to keep Samaritan from destroying The Machine and establishing its hold over everybody.