'''Season 1, Episode 23''':

->''"I did this corporate training thing once -- I was blackmailing the CEO, long story -- but they did this exercise called the trust fall where you close your eyes, and fall, and wait for someone to catch you. [[BatmanGambit I knew you boys wouldn't let me down]]."''

The latest number is Caroline Turing, a discreet psychologist for some of the wealthiest and most dangerous people in New York. One client in particular, a shady lawyer named Hans, continuously implies that something bad will befall Caroline if she blabs any of his secrets. Meanwhile, Fusco has made a name for himself in HR, enough that he's earned a meeting with the inner council -- a city councilman, a high-ranking police officer and a district attorney -- and a place as Officer Simmons' right-hand man. To make up for the lack of income since Elias' arrest, HR has expanded into murder-for-hire and their latest target, on behalf of an anonymous well-paying client, is Caroline Turing. Reese and Finch theorise that one of Caroline's clients wants her dead so she won't expose their secrets, so Reese goes undercover as a patient in order to plant cameras and peek in her diary. Finch tries to track the accounts used to pay HR for the hit, to no avail, so he calls in Zoe Morgan to help figure out which of Caroline's clients are likely to have hired HR. Unbeknownst to all, Alicia Corwin has been shadowing Finch and tracks him to the library.

Simmons' hired goons set their ambush, but Reese foils the attempt and spirits Caroline to a downtown hotel where they can hide out, and HR follows. Finch calls Carter for backup, but she's dragged away by Agent Donnelly and the FBI task force: they've spotted Reese on the CCTV cameras going into the hotel with Caroline (Donnelly assumes she's a hostage) and are dispatching units to hunt him down. HR's goons enter the hotel and Finch directs Reese and Caroline to the roof where they can steal a helicopter and escape, but the FBI gets there first: they land a tactical response team on the helipad. Reese and Caroline are trapped between the FBI and HR; Finch and Carter feed him info on how to stay undetected, while HR plot to blow up an entire floor of the hotel in order to take Reese out, helped by their "man on the inside". Zoe tracks down "Hans", who is actually a lowlife conman that was hired to be Caroline's patient and threaten her. Finch leaves the library to enact a plan to get Reese and Caroline to safety; while he's gone, Alicia breaks into the library and snoops around.

Reese and Caroline make it to the freight elevator, as per Finch's escape plan, but HR's goons attempt to cut them off on level 25. Carter warns Reese in time for him to stop the elevator and he tells Caroline to fiddle with the elevator controls and bypass the floor that HR is on, while the goons plant bombs on level 25 and the FBI come at them from the floor above. Reese and Caroline are out of time and have no options -- which is when Finch enacts his plan to hack a rooftop signal dish and knock out all communications (bar a few select frequencies) in Manhattan, styming the FBI and HR's plans and giving Reese and Caroline a chance to escape. Simmons cuts the hotel's CCTV feeds to cover his tracks but Donnelly sees him doing it just before the cameras go out. Carter sees Fusco texting people and confronts him in the bathroom at gunpoint, accusing him of being HR's man. Fusco owns up to being Reese and Finch's agent in the NYPD, to Carter's surprise, but there's no time to compare notes: Finch calls and tells them both to get downtown and help Reese, and Fusco tells her that thanks to his stint as the mole in HR, he knows who they all are and exactly how to bring them down.

Reese and Caroline reach the subbasement service tunnels; along their escape route, they find Simmons' car, with something interesting in the trunk. As Finch waits in a car at the rendezvous point by the water, Reese sends Caroline ahead while he holds off Simmons and the HR goons -- but it's not Caroline who gets into the car, but Alicia, with a gun, who demands that Finch tell her how to shut down the Machine. She's tired of running, she explains, but Finch tries to convince her it's not the Machine she's been running and hiding from -- and not the Machine that killed Nathan, like she suggests -- but rather the people who control it. He takes umbridge with Alicia's comments about who is and isn't the "wrong type of people" to have access to the Machine, accusing her of hiring HR to kill Caroline specifically to draw him out, but Alicia has no idea what he's talking about. Meanwhile, Reese is pinned down and almost out of ammunition when Carter and Fusco arrive to bail him out. The goons try to escape in Simmons' car (Simmons himself just hides) and Reese, Carter, Fusco pursue, but the chase only goes on long enough for the goons to get out in the open before Reese activates the explosives he hid in the car, killing the goons. And in the city, Zoe breaks into Caroline's office and finds that a lot of the photos and diplomas are fake. When she tries to access Caroline's laptop, the system files auto-delete and leave a single line of computer code: "ROOT" repeated over and over.

"Caroline" shoots Alicia in the head as she gets into Finch's car, introducing herself as Root; she hired HR and placed a hit on herself in order to lure out Finch, likening it to the "trust fall". She tells Finch they have a lot to talk about and forces him, at gunpoint, to drive off. Later, Reese finds Alicia's body on the waterfront. Thanks to Carter and Fusco, Donnelly receives evidence incriminating everyone in HR and the inner council is arrested, but Simmons slips away. Reese returns to the library with no idea what to do next, so he walks out onto a street corner and stares up at a surveillance camera.
->'''Reese''': He's in danger now, because he was working for you. So you're gonna help me get him back.

The Machine processes this for a moment. Then the nearest payphone rings, and Reese answers. Roll credits, and cue screaming fans.
!!Tropes present in this episode include:
* AloneWithThePsycho: A classic example of this trope, with Zoe uncovering the evidence of Caroline's fakery just as Finch discovers that Alicia is ''not'' the one who put out the HR contract.
* BatmanGambit: Root's plan relies on Reese and Finch [[WeHelpTheHelpless doing what they do best]].
* BigDamnHeroes: Carter and Fusco coming to Reese's rescue, when he's cornered in the ParkingGarage and running out of ammunition.
* BoomHeadshot: Root to Alicia.
* CameraSniper: Finch says it's important no-one find out about them. Cut to him being photographed by Alicia.
* CarBomb: Reese turns the sedan the HR cops used into one by breaking into the trunk and wiring up the detonators to some of the explosives they brought with them.
* CharacterDeath: Alicia Corwin is killed by Root.
* CliffHanger: In the proud Person of Interest tradition.
* ContinuityNod
** Reese uses the alias "Rooney" again.
** The street corner where Reese speaks to the Machine is the same as the one in the pilot.
* CutPhoneLines: As the FBI is about to storm the floor where Reese is, and as HR tries to ignite a gas main with the mobile phone I.E.D they've planted, Finch hacks into an emergency protocol that cuts off communication except for authorized phone numbers, the only one belonging to Reese.
* TheDogWasTheMastermind
* DramaticUnmask: Of a form. Rather than the traditional actual mask, Root loses her digital mask and reveals herself to Finch. Not coincidentally this unmasking is combined with her shooting Alicia Corwin, fulfilling the dramatic requirement.
* {{Foreshadowing}}:
** Alicia poses the hypothetical question of what would happen if someone bad was able to access the Machine. Not long afterwards, Finch is kidnapped by just such a person.
** Likewise, before Finch knows who 'Caroline Turing' is, he notes that she's a woman after his own heart. [[DistaffCounterpart He's not]] [[EvilCounterpart wrong....]]
** Alicia ironically refers to the Machine as God. She won't be the last.
* FreezeFrameBonus / ColourCodedForYourConvenience / {{Foreshadowing}}: From the beginning of the episode, "Caroline" has a yellow bracket in the Machine's POV.
* HearingVoices: Spoofed; Caroline isn't happy when Reese starts talking to Finch on his hidden earpiece. Of course, seeing as she's actually Root (and knowing Root's personality from later episodes), she probably secretly found this hilarious.
* HowWeGotHere[=/=]InMediasRes: the episode opens with Donnelly pulling Carter in to his hunt for "The Man in the Suit," then flashes back a couple of days
* IKnowYoureWatchingMe: Reese speaks to a CCTV camera, knowing that the Machine is always watching.
* InstantAIJustAddWater
* InternalReveal: Carter and Fusco finally discover that they've each independently been working with Reese and Finch. Finch discovers the identity of Root, the hired killer from the earlier episode "Root Cause".
* LaserGuidedKarma: The HR cops plan was to blow up a hotel through sabotaging the gas main. Many of them die when Reese activates the explosives he's wired up in the back of their car.
* MakeItLookLikeAnAccident: Caroline's death is meant to looking like a street mugging, then they decide to make it look like the Man in the Suit [[TakingYouWithMe blew himself up when facing capture by the FBI]].
* MasterActor: Root as "Caroline Turing". Only at the end when she leaves Reese does her mask partially slip, before she entirely drops the charade to meet Finch.
* MeaningfulName: "Turing," as in "[[TuringTest Alan Turing]]," given what we find out about her motives in the next episode. Later turns out to be one her signatures in creating aliases.
* MissionControl: Taken to extremes with Finch trying to help Reese and Caroline escape the building, Donnelly trying to guide an FBI SWAT team towards them, Carter helping Reese avoid the SWAT Team, while HR's mole tells their hit team how to avoid the cameras and the FBI SWAT team and find Reese and Caroline.
* MissingMissionControl: At the end of the episode.
* MusicalSpoiler: When Zoe confronts Hans with the fact that he's actually a smalltime conman and he notes he was blackmailed into posing as Turing's client, a few notes of Root of all Evil are played. The full theme comes later when Root drops the mask.
* NarrowedItDownToTheGuyIRecognize: Creator/AmyAcker as Caroline Turing/Root. There's also the factor that watching the show any time after its original broadcast essentially nulls the twist, because of Root's later significance.
* NoKillLikeOverkill:
** HR is willing to blow up an entire hotel floor to kill Caroline and the Man in the Suit.
** Reese kills the members of the HR hit-squad by blowing up their getaway car.
* NotSoDifferent: Again, Finch and Root. Unknowingly lampshaded by Finch.
* ThePictureCameWithTheFrame: During TheReveal, Zoe finds that the pictures in Turing's office are cut from magazines.
* ProphecyTwist
* ProperlyParanoid: Commented on by Caroline when she says she'd diagnosed Reese as paranoid. Turns out Reese isn't paranoid enough.
* RewatchBonus: In John's therapy session, Turing intuits that he's ex-military and asks him whether he feels he has to protect people. Innocent questions perfectly appropriate for a therapist trying to get a feel for her client, but in hindsight, she very probably used exactly those two criteria to find the new guardian angel in her life that appeared after she put out the hit on herself.
* RightHandVersusLeftHand: Carter and Fusco are not amused to find they've both been [[WorkingTheSameCase working for the same side]].
* RoguesGalleryShowcase: HR, Donnelly's FBI task force ''and'' Root.
* SaveTheVillain
* SherlockScan: Caroline picks up that Reese is ex-military and has an urge to protect people, perhaps because of a previous loss. However, it turns out she knows who Reese is, and so she may have known all that from other sources and simply be giving the impression of doing this.
* ShoutOut: The nearby public phone ringing is likely inspired by a famous scene involving an AI contacting the protagonist of ''Literature/{{Neuromancer}}''.
* StandardFemaleGrabArea: Reese to Caroline.
* TemptingFate
** Caroline says to Reese, "You think there's something lurking around every corner" as a HR assassin waits around the corner.
** Finch says they can't afford to have anyone find out about them. Cue CameraSniper from Alicia Corwin.
* ThisIsNoTimeToPanic
* TiredOfRunning: Alicia uses these exact words.
* TitleConfusion: Most places list the episode as "Firewall," but the season one [=DVD=] set consistently refers to it as "Fire Wall".
* VillainousDemotivator: Simmons tells his hit team that as Caroline caught a look at their faces, it's kill her and the Man in the Suit, or be killed to protect the rest of HR from them turning state's witness.
* WhamEpisode
* WhamLine:
** Alicia revealing that she actually wants to destroy the Machine.
** When "Turing" shoots Alicia and slips into the car with Finch.
-->'''Root:''' It's so nice to finally meet you, Harold. You can call me Root.
** [[foldercontrol]]

[[folder: Error: Continuity of operations compromised. Evaluating options... ]]
Followed by the pay phone ringing.
* WhamShot: "ROOT" shows up on Turing's computer.
* WhoAreYou
-->'''Reese:''' Let's just say we both help people with their problems. I'm just a little more hands-on.
* YouShallNotPass: Reese stays behind to battle the HR hit team while Caroline escapes. Of course that's exactly what she wants, leaving her free to kidnap Finch.

[[folder: Error: Continuity of operations compromised. Evaluating options... ]]