On Unix-like systems, / is the root directory or "root path", the alternative title of the episode. In this case, it literally refers to the path the character Root will take to do the Machine's bidding.
Main Plot Points
Under the orders of the Machine, Root stops a prisoner transfer bus and removes Billy Parsons. With the help of Daizo, Root is able to create a fake identity with which Parsons, who is able to forge a German man's signature and accent, is able to use to intercept a note.
Root orders Daizo to meet Daniel Casey in Perth, Australia and sends Billy back to jail with advice from the Machine on how to protect himself.
Root is next directed to Cyrus Wells, a janitor working in the building on the note's address. Cyrus believes there is a reason for everything and doesn't question Root too heavily on her actions.
In the park, Root meets Finch who had received Cyrus' number the moment Root approached him, leading Finch to believe that the threat to Cyrus is Root's disregard for collateral damage. Root reveals she is working to save the world from the threat of Samaritan which is being rebuilt and is nearly finished. Before departing, Root sends Finch a spam email and gives him mysterious directions.
Reese and Shaw follow Root only for Root and Cyrus to come under attack by Peter Collier and Vigilance. Reese and Shaw take out the Vigilance members while Finch decodes the message hidden in the spam revealing that Vigilance is after Cyrus due to intercepting a communication revealing that he is vital to a Decima Technologies plan to rebuild Samaritan, confirmed when John Greer arrives on the scene.
To protect Cyrus, Root has him arrested by the police and Finch contacts Fusco to protect him. After Fusco gets the charges against Cyrus cleared up, Root arrives in the guise of an FBI agent with a warrant to take Cyrus, warning that there are many bad guys after Cyrus and Fusco is unable to stop her.
In the park, Decima operatives use a jamming device to cut Root's connection to the Machine and attack her and Cyrus. Root is rescued by Reese but Decima gets away with Cyrus.
Root questions why Finch made the Machine to care so much about people.
Root takes Finch's device, a cochlear implant to a doctor who installs it. With the implant Root is able to communicate directly with the Machine without needing a phone.
Root arrives with her new implant preventing Decima from jamming her connection to the Machine. Reaching the lab, Root chooses to save Cyrus rather than the chip.
As Reese informs Finch of their failure to stop Decima, Shaw returns as Finch fears for her safety.
Root helps to set Cyrus up with a new life.
Finch accepts Root as an ally.
After the theft of the chip, the Machine begins calculating the threats Samaritan poses and calculates Samaritan as being over 50 percent completed.
Root refers to Reese as Shaw's "Australopithecine co-worker." Australopithecus is the last stage of pre-erect development of early homo sapiens as well as several other hominids. It is one of two primate references Root uses regarding Reese.
The device surgically implanted behind Root's ear is a cochlear implant, which is typically used for post-lingual adults with sensorineural (nerve) deafness. Root's hearing loss, on the other hand, is due to the loss of one of the conductive bones in her ear, a conductive hearing loss. Regardless, Root would not regain hearing in her right ear with the implant, but rather be able to hear limited sound.
Root uses the alias Augusta A. King on her FBI badge. King, also known as Ada Lovelace, was a British mathematician known for her work on early mechanical general-purpose computer, the Analytical Engine.
Root uses cayenne pepper poured in hot oil to aerosolize the capsaicin (irritant) in the chile, creating instant tear gas. The Chinese used this method to fight off enemies more than two thousand years ago.
According to the Machine's probability analysis, a mass casualty event has a 12.12% chance of happening at [40.71448/-74.00598], or just near New York City Hall. This foreshadows the events of the season finale.
According to the receipt Root picks up, the episode takes place on March 11, 2014.
"/" was originally titled "Entanglement".
Bloopers and Continuity Errors
Root mentions being at Stoneridge Psychiatric Facility to Billy Parsons, but accidentally refers to it as "Ridgestone".
In the scene where Root was entering the building at 7:04 AM, she was walking and talking to the Machine. In the surveillance Finch reviews, she walked directly up to the POI at 7:04 AM.
When Shaw fights off Vigilance operatives using a metal pole, she is wearing flat-soled boots. After the fight, she returns to the library carrying the metal pole, but she is wearing high-heeled boots.
Root learns the hard way that her "analog" interface with the Machine can be jammed.
The Machine also presents pictures of events previously seen in other episodes: Trent Russell being mugged after Root set him up in “Bad Code”, and Alicia Corwin after Root killed her in “Firewall”. The Machine also found a picture of Cyrus Wells and his two colleagues whom Root murdered in 2009. The image would appear again later in the episode.
Cyrus Wells became a person of interest at the exact time that Root approached him.
The Machine predicts that Samaritan is a threat to Fusco, Control, Collier, Hersh, Shaw, Reese and Finch. It also predicted the probability of their deaths at 10.7%, 11.5%, 21.4%, 30.7%, 49%, 56.9% and 68%, respectively.
Finch's line to Root "Everyone is relevant to someone." was originally said to him by Nathan Ingram in 2009, the day before the Machine was turned over by IFT, after Finch refers to the irrelevant list as "drawing a line somewhere". (“No Good Deed”)
"Did you not hear about my chat with Control? She's fun... in an unnecessary stapedectomy kind of way. But I do miss music in stereo." (Root, to Finch)
"I know it seems weird, but I'm one of the good guys now, Harold." (Root, to Finch)
"I couldn't make you look bad if I tried." (Root, to Shaw)
"Wanna see who's got better aim?" (Shaw, to the sniper)
"Now that you're not going to die, we can have a little chat." (Shaw, to the sniper)
"Are you guys going to shoot again, or just talk me to death?" (Shaw, to Collier)
"In the words of Benjamin Franklin, join or die." (Collier, to Shaw)
"Joined the Bureau last month to get access to a nuclear plant in Tulsa. Long story." (Root, to Fusco)
"What was I supposed to say? 'Sorry, boss, Agent King is actually a superpowered nutball. Just ask my buddy, the urban legend'?" (Fusco, to Reese)
"You're right, about all of it. I did work for the government, and I do want revenge. But if that work taught me anything it's that how you do matters as much as what you do. And by that metric, you're all just terrorists. And I kill terrorists." (Shaw, to Collier)
"If anyone would know about hubris, it would be the man who built God." (Root, to Finch)
"Everyone is relevant to someone." (Finch, to Root)
"That's the problem with humans... they just sit around, hoping that someone will fix things. But no one will. No one cares. The universe is infinite and chaotic and cold. And there has never been a plan. At least not till now." (Root, to Finch)
"How badly did you have to break it to make it care about people so much?" "That didn't break it. It's what made it work. It was only after I taught the machine that people mattered that it could begin to be able to help them. I'd like to do the same thing for you, if you'd let me." (Finch and Root)
"In the first 30 minutes after Samaritan comes online, a lot of people are going to be killed. But they're gonna start with four: You, me, your helper monkey, and Shaw. And there won't be a damn thing anyone, including your machine, can do to stop it. You think I don't care about people, Harold? I'm doing all of this to save you." (Root)
"I love it when you play doctor." (Root, to Shaw)
"Hey, Cuckoo's Nest, the meter's running." (Fusco, to Root)