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201The Contingency” (Finch) 209C.O.D. 217Proteus
202Bad Code” (Root) 210Shadow Box 218All In
203Masquerade 2112πR 219Trojan Horse
204Triggerman 212Prisoner's Dilemma” (Reese) 220In Extremis” (Fusco)
205Bury the Lede 213Dead Reckoning” (Stanton) 221Zero Day” (Finch/Ingram)
206The High Road” (Finch) 214One Percent” (Finch/Ingram) 222God Mode” (Finch/Hersh)
207Critical 215Booked Solid
208Til Death” (Finch) 216Relevance
Zero Day

POI 0221 Main.png



Air dateMay 2, 2013

Running time43:45

Production code2J7221

Written byAmanda Segel &
David Slack

Directed byJeffrey Hunt


Images (54)

Can you hear me?

— The Machine, to Reese

"Zero Day" is the 21st episode of season 2, and the 44th produced hour of Person of Interest. It originally aired on May 2, 2013. 


Reese and Finch realize that a virus has rendered The Machine erratic, causing it to issue numbers irregularly. As they race against the viral countdown, the team searches for a tech millionaire so elusive no one has ever seen him. Also, Detective Carter's mission to bring Cal Beecher's murderer to justice lands her squarely within the sights of the HR criminal organization.

Origin of the Title

The episode draws its name from the technological term a zero-day attack, an attack (or threat) that exploits a previously unknown vulnerability in computer software. Because the vulnerability is unknown, it leaves software engineers with no time to fix the weakness before the attack. The attack gets its name given there are zero days between when the vulnerability is realized and when the hack happens, leaving zero days to fix it before any damage is done. The flashback to the day before Finch proposes to Grace plays with the title when Finch declares it Day Zero of his new life.

Main Plot Points

  • Reese complains of no new numbers for ten days.
  • Special Counsel is concerned with no new relevant numbers and through Alicia Corwin's chip tells his superiors that he suspects Decima Technologies of the attack on The Machine.
  • Finch receives the number for Ernest Thornhill who is a composite and who employs people to copy code from the previous day back into the system.
  • Root reveals her false identity to Special Counsel and gets information regarding Thornhill and offers Finch a collaboration.
  • After an attempt on Thornhill by Decima, Finch realizes Thornhill is the Machine.
  • The Machine resets for the first time since going online, which initiates "God Mode" giving Admin access to whoever answers the phone for 24 hours. When Root answers the phone Harold splits the junction so that two phones ring and Reese is also given access.
  • Greer reveals that Finch is the original source of the virus and The Ordos Laptop to both Reese and Shaw.
  • Carter continues her investigation into Cal Beecher's murder, leading HR to try to murder and then frame her.
  • Root forces Finch to help her in her attempts to save the Machine from Decima.
  • Both Reese and Root receive a call that grants them full administrative access to the Machine for 24 hours.


In the flashbacks, Finch tells Nathan of his plans to marry Grace and Finch discovers Nathan Ingram in the Library working the irrelevant list.

Episode Notes

  • Similar to “Relevance”, the title sequence is interrupted partway through as the Machine is beset by the virus.
  • The content of the Machine's memory, along with its own "identity", are deleted every night at midnight. 1.618 seconds later, it reinstantiates, completely new. Finch devised this daily function as a way to prevent the Machine from evolving, after he began to encounter anomalies and realized the Machine was imprinting on him like a child with a parent ("It started looking out for me, altered its own code to take care of me. It was behaving like a person. But the world didn't need a person to protect it. It needed a machine.") Despite this, the Machine demonstrates an instinct for self-preservation by creating the Thornhill identity as a means to store its memories through "an external hard drive made up of people and paper". The Machine's memories are printed every day before the midnight erase and then typed back in the day after by the employees at Thornhill's data entry company.
  • Finch says the Machine deletes itself and "reinstantiates 1.618 seconds later". 1.618 is not a random number but rather phi (ɸ), a "golden ratio" that is widely present in nature, where it creates Fibonacci spirals. It is commonly used in architecture and arts, where it creates aesthetically pleasing proportions.
  • At the end of The Machine's reboot process, it generates four lines of binary code. When it's converted to ASCII characters the phrase "Quis custodiet ipsos custodes" is revealed.
    Screenshot zero day binary.png
    This Latin phrase is translated as "Who will guard the guards themselves?", a variation on the old axiom "Who watches the watchers?" In other words, who is watching the people who watch us all?
  • Nathan Ingram was placed in the irrelevant list, foreshadowing his death.
  • "Zero Day" takes place over the course of Day 4138 of the Machine's operation, or April 30, 2013.

Acronyms and Vocabulary

  • NOC (Non-official Cover): an identity assumed by a spy that is not tied to their government. Although such an identity may be officially sanctioned, it leaves the spy without the protection of diplomatic immunity, and if caught, makes them subject to criminal action by the country in which they are active.

Production Notes

  • In a DVD featurette following one day's production of this episode, Michael Emerson notes that the flashback in which Finch follows Ingram from a cafe to the Library is the first in which Finch wears his now-trademark vest. This marks the transition from Finch's old, happy life to his new secretive life to come.


  • The town car that is blown up while supposedly carrying Thornhill has two different styles of NY license plates, front and back.


  • The failed bomber mentioned by Special Counsel (and projected on his monitor), may refer to the Nigerian underwear bomber story, which took place aboard a flight from Amsterdam to Detroit in December 2009.
  • Finch reroutes the Machine's call to its admin by accessing the telephone junction box. Once the call was established, he spliced the lines in order to create a "party line", where two or more subscribers share the same phone number, so that two persons can co-administer the Machine. Party lines are seldom used now, but were common in the early days of the telephone service, when phone usage was cost-prohibitive and phone lines were scarce. 
  • Root claims she is not a sociopath. A sociopath is an individual who exhibits anti-social behavior, or who may act without conscience or a sense of remorse. 


Main article: Zero Day/Memorable quotes                                                 
  • "The Machine is being buried under an avalanche of false data, but somehow it was able to get us his number." (Finch)
  • You don't want to get married under another one of your pseudonyms. You don't think she will consent to be Mrs. Ostrich? (Ingram)
  • "I've seen it dozens of times before. A NOC sets up a business, residence, a whole life. Then the operation gets scrapped. No one cleans up the fake ID, and you wind up chasing empty town cars around the city." (Reese)
  • "So if you're like me, and we both know you are, you designed the Machine so that a catastrophic crash puts it into a remote debugging protocol; a God mode that gives the admin full access to all of its data." (Root)
  • "Five and a half months ago, [Thornhill] opened this account with a single penny. Twenty-four hours and thousands of microtransactions later, his balance had ballooned to just over twenty million dollars. And he immediately began trying to buy up pay phone companies all over the city." (Finch)

Episode Opening

The episode begins with a series of flashbacks consisting of brief excerpts from key scenes from the second season which revisit what happened previously:

"I was able to crack the virus' code and learn the name: Decima Technologies." (Finch) (“Trojan Horse”)
"...I believe they created the virus to infect a single target: the Machine."(Finch) (“Trojan Horse”)
"Unlike the CIA, my employers have an inkling of what happens next." (Greer) (“Dead Reckoning”)
"I just thought it needed an off switch just in case." (Ingram) (“No Good Deed”) (Season 1)
"That storm on the horizon that I mentioned. I'm afraid it's arrived." (Finch) (“In Extremis”)

External links